Tamper-protected hardware and method for using same

ABSTRACT

One of the various aspects of the invention is related to suggesting various techniques for improving the tamper-resistibility of hardware. The tamper-resistant hardware may be advantageously used in a transaction system that provides the off-line transaction protocol. Amongst these techniques for improving the tamper-resistibility are trusted bootstrapping by means of secure software entity modules, a new use of hardware providing a Physical Unclonable Function, and the use of a configuration fingerprint of a FPGA used within the tamper-resistant hardware.

FIELD OF THE INVENTION

The invention relates to various techniques for improving thetamper-resistibility of hardware. The tamper-resistant hardware may beadvantageously used in a transaction system that provides the off-linetransaction protocol as described in the priority applicationPCT/EP2011/001219 of the same applicant. Amongst these techniques forimproving the tamper-resistibility is trusted bootstrapping by means ofsecure software entity modules, a new use of hardware providing aPhysical Unclonable Function (PUF), and the use of a configurationfingerprint of a FPGA used within the tamper-resistant hardware.

BACKGROUND OF THE INVENTION

Security is nowadays globally known of being in the need to get enforcedby measures of authentication and change-protection incorporated andembedded into chips by fitting to physical and individual descriptorsand characteristics not able to get copied. Such a mechanism is providedby a PUF (Physical Unclonable Function). This feature is called a(physical) fingerprint (FP) based on different physical effects andvariables.

But even in case of this kind of protection, such a system is not inprinciple offering protection against an individual attack: Through(unrecognized) intercepting of data lines, a PUF-based secret (thefingerprint) can be disclosed and reconstructed on the digital layer. Ifthis is successful, than the PUF can—depending on the implementation—bebypassed, if necessary with the help of a separate digital storage. Suchan attack could be the more successful; the more the (IC) applicationdevelopers are building their systems on the merits of being technicallyand functionally in a good shape but not taking notice of guidelines forcriteria of security and cryptographic integrity.

Today most (software) security mechanisms are found in hardened(embedded) systems based on hardware security functions. Processingelementary units like microprocessors and controllers are responsiblefor performing safety-critical tasks on digital networks, in PCs, mobilephones, and other handheld (wireless) devices, Internet-based TVs orcars. Any failure of these systems could have an immediate impact on thereal life.

State-of-the-art standards covering physical security requirements forcryptographic modules can be found in the highest level of securitydefined in the US standard NIST FIPS POB 140-2 Level 4 (available athttp://www.nist.gov/manuscript-publication-search.cfm?pub_id=902003,page 3): “At this security level, the physical security mechanismsprovide a complete envelope of protection around the cryptographicmodule with the intent of detecting and responding to all unauthorizedattempts at physical access”.

Due to the high number of security problems it can be assumed that theexisting applied solutions are not as good as they should. Their stateof security seems not sufficient for a suitable protection; the currentapproaches suffer either from the non-existence of on-a-big-scale“secure-enough” usable architectures or from difficulties in theirreliable protection against ((semi-)invasive) attacks intendingcounterfeit (e.g. plagiarism) chips. But not only the protection ofapplications is an issue, there is also the wish to make malicious (e.g.Trojan horse with backdoors) of well-known leader-referenced (and allothers too) chip art impossible. IC metering (midnight overproduction)is another revenue cutting aspect resulting into massive losses of jobs.

Jarrod A. Roy et al. introduced a way for IC protection (J. A. Roy, etal., “EPIC: Ending Piracy of Integrated Circuits”, Design, Automationand Test in Europe, 2008. DATE '08, pp. 1069-1074, 2008) requiring thatevery chip be activated with an external key, which can only begenerated by the holder of the IP rights, combiningautomatically-generated chip IDs with public-key cryptography. Statingand rating potential attacks and countermeasures they state: “ . . . thepirates must modify chips or masks . . . ”. Such a modification has thanto bypass the security logic in the chip. They conclude: “However, thisscenario is unlikely because, at 45 nm and below, masks are much harderto read than the actual shapes on the chip, due to ResolutionEnhancement Techniques (RET). Scanning the actual shapes in silico iseven harder, and the investment required for this may not pay offbecause pirated chips sell at a lower cost, often at low volumes”. Thiscould be a deceptive cadence.

Traditional physical security coded in transistors, gates, memory cells,or conducting paths gets more and more devaluated due to theavailability of de-capsulation and de-processing knowledge (e.g. etchinglayers away), tools, and services (e.g. drilling holes or micro-probingusing FIBs). Today even 3D-shaped, multilayered SoCs are de-layered,imaged, and de-layouted on a professional way. With that in hand, atransistor level netlist, gate netlist, and at least the completehierarchal schematics are (cheap) reverse engineered. This put heavypressure on the demand for a security technology not susceptible to suchattacks. With this in mind, the scientific community searches for waysto achieve that.

First publications about the usage of physical properties gained fromdisordered systems for cryptographic applications could be traced backto Bauder (D. W. Bauder, “An anti-counterfeiting concept for currencysystems,” Sandia National Labs. Albuquerque, N. Mex. 1983) and Simmons(G. Simmons, “A system for verifying user identity and authorization atthe point-of sale or access,” presented at the Advances inCryptology-EUROCRYPT '88, 1984). The word POWF (Physical One-WayFunction) and PUF (Physical Unclonable Function) were coined by PappuSrinivasa Ravikanth (S. R. Pappu, “Physical one-way functions,”Massachusetts Institute of Technology, 2001 and S. Devadas, et al.,“Design and Implementation of ‘Unclonable’ RFID ICs forAnti-Counterfeiting and Security Applications,” RFID World 2008, 2008).PUFs do not embody a direct accessible memory, they implementchallenge-response authentication: A physical stimulus (calledchallenge) is applied to the PUF, resulting into a reaction (calledresponse). A specific challenge and its corresponding response togetherform a Challenge Response Pair (CRP). PUFs should be unclonable withintwo aspects: Physical means the infeasibility to construct a PUF copyproducing the (all) same CRPs of the original one. Mathematical meansthe infeasibility to describe a PUF in a formal logical way like afunction or algorithm. It is the combination of both attributes whichrenders a PUF truly unclonable.

PUFs use randomness that can be intrinsically or explicitly introducedby a physical system. PUF relevant effects are found nearly everywherein the matter-based world. Representatives of explicitly introduced PUFsincludes optical ones using unique speckle scattering pattern, coatingones randomly doped with dielectric particles resulting into anunpredictable capacitance, or RF susceptible resonator circuitry buildon integrated capacitors and coils providing strong resonance peakscalled LC-PUFs. A fascinating work is theRadio-Frequency-Certificate-Of-Origin (RF-COA) approach presented byDarko Kirovski, Gerald Dejean et al. (V. Lakafosis, et al., “RFFingerprinting Physical Objects for Anticounterfeiting Applications,” inMicrowave Theory and Techniques, IEEE Transactions on, 2011, pp.504-514; V. Lakafosis, et al., “An RFID system with enhancedhardware-enabled authentication and anti-counterfeiting capabilities,”in Microwave Symposium Digest (MTT), 2010 IEEE MTT-S International, 2010and V. Lakafosis, et al., “RFID-CoA: The RFID tags as certificates ofauthenticity,” in RFID (RFID), 2011 IEEE International Conference on,Orlando, Fla., 2011). The basic idea covers the construction ofunclonable tags and labels containing random constellations ofscatterers printed on paper (one implementation) or laminated intocredit-card (sized) applications consisting of “ . . . extremelydifficult to replicate, random arrangement of a conductive material,such as copper wire, mixed with a firm dielectric material, such asplastic PET mold, that produces a unique and repeatable response in thenear-field.”. This serves then as genuine-identifying-oracle, the samebut analog direction as intended with the RFID (far-field) technologybased on digital properties. RF-COAs do not contain any electroniccircuitry and are challenged using an external (trusted) RF-reader.

A more ‘exotic’ representative is the magnetic PUF as found on somemagnetic stripe cards fabricated by blending particles of barium ferriteshaped in many different sizes together in slurry. After drying up, thereceptor layer is sliced into strips and applied to plastic cards. Thisattaches a distinctive, repeatable, and readable magnetic signal. Such asystem was the technology ground for a system named Magneprint developedby Ronald Indeck at Washington University (R. S. Indeck and M. W.Muller, “Method and Apparatus for Fingerprinting and AuthenticatingVarious Magnetic Media,” 1999). Examples of PUFs based on intrinsicrandomness are silicon ones, e.g. bi-stable logic memory cells likeSRAM-PUFs and Latches, Flip-Flops and Butterflies based ones, delaybased ones, e.g. MUX/arbiters, ring oscillators or glitch PUFs to name afew. Intrinsic PUFs are said being attractive because they can beincluded in a design without or with only little modifications to the(silicon) manufacturing process. Most interest about intrinsic PUFs iscaptured by CMOS-based ones. The enrollment and verification phasehandling of CRPs can be augmented through Fuzzy Extractors (FEs) (alsocalled Helper-Data algorithms) realizing three functionalities: Errorcorrection for information reconciliation, randomness extraction forprivacy amplification, and robustness protecting integrity. FEs shouldalso be seen in the light of information extraction, noise cancellation,and data compression: Characterization of the PUF and formatting theHelper-Data structure during the enrollment plus extracting andprocessing the physical data during the verification, (re)generating thefingerprint. This enables a number of new applications such asPhysically Obscured Key (POK) storage where the control layer derives asecret from the PUF.

Trying to cover the physical complexity of a PUF structure from thealgorithmic point of view, the Kolmogorov Complexity (KC) can be used.It describes in our PUF case the randomness or entropy a physical systemhas, defined as the size of the smallest computer program (in bits)required to generate the object in question to some degree of accuracy.KC stresses—in contrast to the Shannon entropy—the measurement ofdisorder without any need for probabilities. Of course, the average ofKC is the same to the statistical entropy for a thermodynamic ensemble.To conclude, PUF's are pointing a way to handle the thermodynamicentropy of an isolated physical system useful for cryptographic elementapplications.

SUMMARY OF THE INVENTION

One of the objects of this invention is to provide a design of atamper-protected semiconductor module usable for transferring electronictokens between peer-devices.

Another object of the invention is to suggest designs for increasingtamper resistance of hardware (e.g. a device, semiconductor module orhardware module).

One aspect of the invention is the provision of a tamper-protectedhardware, which is inter alia suitable for use in the transactionsystem. In this respect, aspects of the invention focus on thefunctionality of the tamper-protected hardware (which may of course alsodepend on its firmware), and on the design of the tamper-protectedhardware so as to avoid misuse and cloning of the tamper-protectedhardware and transaction system. It will become apparent from thefollowing that there is often also a synergy between functionality anddesign concepts, which in combination improve the robustness of thetransaction system.

As to the functionality of the tamper-protected hardware, same may beprovided by the firmware (e.g. by means of TRUSTLET(s), and/or includingintegrity checking functions—see below), or may be provided in somededicated hardware of the tamper-protected hardware. As some of thesecurity measures to improve the tamper resistance of thehardware/device on the chip level and others provide tamper protectionon the packaged chip level it will be referred to (tamper-protected)semiconductor modules and/or (tamper-protected) hardware modules. Asemiconductor module is denoting the packaged hardware module (chip). Inone exemplary implementation, a semiconductor module is obtained bybonding a hardware module on a circuit carrier and packaging the bondedhardware module into a casing box. A hardware module could be, forexample, implemented as a chip, a die or a multi-die.

A tamper-protected semiconductor module may also be referred to as aHardware Security Module (HSM).

In one exemplary embodiment, the tamper-protected semiconductor moduleis a tamper-protected chip, die or multi-die. In another exemplaryembodiment of the invention, the tamper-protected module is implementedin a tamper-protected integrated circuit (IC). The tamper-protected ICmay be implemented as a tamper-protected die having a tamper-protectedenclosure.

A further aspect of the invention is related to the design oftamper-protected hardware. The tamper-protected hardware describedherein may be usable in a transaction system as for example described inthe priority application PCT/EP2011/001219 of the same applicant, but isnot limited to such use. The various designs of tamper-protectedhardware according to this invention are not limited to any specificapplication, but could be used to “harden” virtually any hardwareagainst tampering. This includes hardening/annealing the hardwareagainst tampering on the hardware/device and/or semiconductor moduleand/or hardware module level.

In this context, it is a further aspect of the invention to provide atamper-protected hardware that is provided with a hardware structureproviding a Physical Unclonable Function (PUF). A set of knownchallenges and their respective corresponding responses of the hardwarePUF is used to verify integrity of the tamper-protected semiconductormodules: Basically, the hardware structure implementing the PUF could bechallenged by one or more challenges for which at least one response ofa challenge is known, and in case the tamper-protected hardware is a“cloned” tamper-protected hardware, the PUF response provided by thehardware structure would not match the expected and known response(s).

For example, in one exemplary embodiment of the invention, atamper-protected hardware comprises a hardware structure providing aPUF—referred to as “hardware PUF” below. The hardware structure providesa response to an input challenge to the PUF implemented in the hardwarestructure. Furthermore, the tamper-protected hardware may be equippedwith a processor unit for providing at least one challenge to thehardware PUF, for which the correct PUF response by the hardware PUF isknown. The processor unit receives a respective PUF response for eachchallenge provided to the hardware PUF, and verifies integrity of thetamper-protected hardware by checking whether the response to the atleast one challenge for which the correct PUF response is known ismatching the response received from the hardware PUF.

Another possible way to use the hardware PUF for securing thetamper-protected hardware module is its use in a segmentation process ofsecrets (e.g. electronic tokens, encryption keys such as for example asecret key, a private key as part of a public key-pair, certificates,etc.) performed by the tamper-protected hardware module: A secret issplit into two parts using a so called “constructor function” or “keygeneration function”. The construction function uses the hardware PUF(this equals to the fingerprint of the PUF) to produce a file calledHelper-Data, which is one of the parts of the secret. Due to the uniquenature of the hardware PUF, the Helper-Data is also unique like anindividual lock and it's key. The original secret can only bereconstructed using the (correct) hardware PUF as well as the correctHelper-Data. Such procedure avoids the (need of) storing of the secretin a digital form on the device as only the Helper-Data is stored. TheHelper-Data do not need to be protected. The secret is extracted fromthe device by using the constructor function only if required. Thesecret, once used, can be removed from all internal (semiconductor)registers and memories.

Another aspect of the invention related to the design oftamper-protected hardware (which may be used together with the hardwarePUF functionality or independent therefore) is to provide atamper-protected hardware module with a FPGA that has a givenconfiguration of an array of processing-elements to provide a givenfunctionality. This given configuration of processing elements can beconsidered as fingerprint of the programmed FPGA. Moreover, a storageunit of the tamper-protected hardware module may store signedconfiguration information indicating the initial configuration of thearray of processing-elements of the FPGA of the hardware module. Thehardware module's processor unit could then verify integrity of thehardware module by verifying integrity of the initial configurationinformation of the FPGA based on its signature and comparing theverified initial configuration information of the FPGA with the currentconfiguration information of the processing-elements of the FPGA.

Another aspect of the invention is also related to the design of thetamper-protected semiconductor module. Please note that this aspect isindependent of the other aspects of the invention provided herein,but—in some embodiments of the invention—the tamper-protectedsemiconductor module may be used in combination with them to furtherimprove tamper resistibility. The basic approach underlying this aspectof the invention is to design a semiconductor module which has uniqueproperties with respect to the deflection of a physical measurand inresponse to an excitation. These unique deflection properties areintroduced by the production process of the semiconductor module and arethus capable of uniquely identifying the semiconductor module. Further,tampering of the semiconductor module will alter its deflectionproperties, which makes tampering attempts observable. Therefore, thesemiconductor module is referred to as “tamper-protected”.

In one exemplary embodiment of the invention in line with this aspect,there is provided a tamper-protected semiconductor module comprising atamper-protected hardware module. The hardware module comprises one ormore emitters for emitting a predetermined excitation that can bemeasured as one or more physical measurands. Further, the hardwaremodule may comprise one or more sensors for sensing the (back-) scatterof the predetermined excitation in form of the physical measurand(s) ina contactless and/or contact-based manner. The tamper-protectedsemiconductor module further has a cocoon that houses the hardwaremodule. This cocoon may also be referred to as a package or housing. Thecocoon is configured to influence the (back-) scatter of thepredetermined excitation to be sensed by the sensors.

The deflection properties of the inside of the cocoon and thus also the(back-) scatter of a given excitation depends on the production process,e.g. the packaging process of the hardware module. Therefore, themeasured (back-) scatter of the predetermined excitation influenced bythe cocoon (deflection properties depending on the production process ofthe cocoon) provides a PUF specific to the tamper-protected hardwaremodule. This PUF function of the tamper-protected semiconductor moduleis also referred to “cocoon PUF” for simplicity. Using this cocoon PUF,the hardware module—according to another more detailed embodiment of theinvention—is able to detect any tampering of the tamper-protectedsemiconductor module, including non-invasive (or passive),semi-invasive, and full-invasive attacks to access the inside of thecovering cocoon, respectively.

Another aspect of the invention is to control a tamper-protectedhardware to boot in a controllable operating environment. Many attackson tamper-protected hardware succeed by managing the attacked hardwareto boot into a non-secure operating environment allowing the executionand/or exploit of the hardware. To prevent such type of attacks, theinvention introduces so-called TRUSTLETs which can be consideredintegrity protected entity blocks (e.g. an entity block contains a codeblock and optionally a data section (e.g. comprising static data and/orinitialization data or the like)) of software/firmware functionality tobe executed by the tamper-protected hardware. The integrity of eachTRUSTLET to be executed, e.g. within the bootstrapping of atamper-protected hardware module, is verified prior to its executionusing a public key infrastructure, so that the execution of maliciouscode is prevented.

According to one exemplary embodiment of the invention, a method forgenerating a software module (i.e. TRUSTLET) is provided. An entityblock to be included to the software module is provided. The entityblock can be considered executable piece of software/firmware (e.g.providing a given functionality). A fingerprint of the entity blockusing a hash function is generated. The fingerprint allows theverification of the integrity of the entity block. The fingerprint isthen encrypted using the private key of a public key pair, to therebygenerate a digital signature of the entity block. The entity block isthen combined with the encrypted fingerprint to form an integrityprotected entity block. Optionally, the entity block may be compressedprior to combining it with the encrypted fingerprint. Furthermore, theintegrity protected entity block is encrypted using a random secret keyto thereby form an encrypted and integrity protected entity block. Therandom secret key is also encrypted using the private key of a publickey pair. The software module (i.e. TRUSTLET) is assembled (orgenerated) by combining the encrypted and integrity protected entityblock, and the encrypted random secret key.

Optionally, each entity block that is included into a TRUSTLET mayitself comprise functionality to execute another TRUSTLET, including theverification of the other TRUSTLET's integrity.

Furthermore, according to another embodiment of the invention, there isprovided a method for bootstrapping a host device/hardware by executinga set of software modules (i.e. TRUSTLETs) in a given order. Thebootstrapping may include:

-   -   a) decrypting an encrypted random secret key of one of said set        of software modules to be executed next,    -   b) decrypting an encrypted and integrity protected entity block        of the software module to be executed next, using the decrypted        random secret key, to thereby obtain an integrity protected        entity block of the software module to be executed,    -   c) decrypting an encrypted fingerprint comprised in the        integrity protected entity block of the software module to be        executed next, and    -   d) generating another fingerprint of the entity block using the        same hash function as used to generate the encrypted        fingerprint,    -   e) verifying the integrity of the entity block of the integrity        protected entity block of the software module to be executed        next by comparing the fingerprint decrypted in step c) with the        fingerprint generated in step d), and    -   f) only in case the integrity of the entity block of the        software module to be executed next is successfully verified,        executing the entity block of the software module to be executed        next,        wherein steps a) to f) are performed for each software module of        the set of software modules thereby realizing a chain of trust.

A further aspect of the invention, which can be considered to be afunctional and design aspect of the tamper-protected hardware, isrelated to providing a validation process of the tamper-protectedhardware, which could be considered an “initialization” of thetamper-protected hardware. This validation process/initialization of thetamper-protected hardware may be performed at a Root CertificationAuthority and provides the tamper-protected hardware module with acertificate that is used as the “identity” or identifying descriptoritem (e.g. a serial number) of the tamper-protected hardware in anytransaction performed. Any tamper-protected hardware that does notundergo the validation process/initialization at the Root CertificationAuthority and that has thus not obtained “its” certificate from the RootCertification Authority will be unusable in the target system of use.

BRIEF DESCRIPTION OF THE FIGURES

In the following the invention is described in more detail in referenceto the attached figures and drawings. Similar or corresponding detail inthe figures is marked with the same reference numerals.

FIG. 1 shows a block diagram of a tamper-protected hardware module(CASTOR) according to an exemplary embodiment of the invention,

FIG. 2 shows a generation process of a TRUSTLET according to anexemplary embodiment of the invention,

FIG. 3 shows the process for validating a TRUSTLET prior to itsexecution according to an exemplary embodiment of the invention,

FIG. 4 shows an exemplary trusted bootstrapping of a CASTOR usingTRUSTLETs according to an exemplary embodiment of the invention,

FIG. 5 shows a comparison of a Read Only Memory (ROM), a Crypto-Functionand a Physical Unclonable Function (PUF),

FIG. 6 exemplifies the PSSBI approach provided by the API Filter of theCASTOR's firmware in combination with the hardware according to anexemplary embodiment of the invention,

FIG. 7 exemplifies the logical PSSBI filter interface functionalismaccording to an exemplary embodiment of the invention,

FIG. 8 shows an exemplary crypto-coprocessor implemented as areconfigurable array of Processing-Elements and switch-boxes (FPGA),

FIG. 9 shows a single processing element of the exemplarycrypto-coprocessor,

FIG. 10 exemplifies the operation of a switch-box interconnectingprocessing element,

FIG. 11 shows an exemplary tamper-protected semiconductor moduleprovided with a cocoon PUF as chip carrier and/or housing, also known asa chip container or chip package,

FIG. 12 shows an exemplary embodiment of a hardware module containingsensor elements and emitters for an excitation comprised by thetamper-protected semiconductor module of FIG. 11,

FIG. 13 exemplifies the separation of a secret by a Helper-Datageneration function based on the secret and the PUF data (fingerprint)provided by a PUF, and

FIG. 14 exemplifies the reconstruction of the secret by a secretgeneration function based on the Helper-Data and the PUF data(fingerprint) provided by a PUF,

FIG. 15 shows an exemplary trusted bootstrapping model covering theserial chain strategy of a CASTOR using TRUSTLETs according to anexemplary embodiment of the invention,

FIG. 16 shows an exemplary trusted bootstrapping model covering thecentral chain strategy of a CASTOR using TRUSTLETs according to anexemplary embodiment of the invention, and

FIG. 17 shows an exemplary trusted bootstrapping model covering a hybridform variation strategy of a CASTOR using TRUSTLETs according to anexemplary embodiment of the invention,

FIG. 18 shows an exemplary digital electronic Wallet (eWallet) accordingto an exemplary embodiment of the invention,

FIG. 19 shows how the concepts of physical effects affect the presentedcocoon PUF mechanisms,

FIG. 20 shows an exemplary principle measurement setup as used withinthe acquisition of measurands in the time domain by using radiofrequency pulses (exemplary shown with reference to Gaussiannano-cycles),

FIG. 21 shows an exemplary block diagram of a CASTOR (also referred toas “nano-Safe”), according to an exemplary embodiment of the invention,including a cocoon PUF, and

FIG. 22 shows a cross-sectional view of a tamper-protected chip based onan exemplary BGA packaging according to an embodiment of the invention.

DETAILED DESCRIPTION

In the following different aspects of the invention will be describedwith respect to various exemplary embodiments that will at least in partrefer to the Digital Cash/eCash context, i.e. the transfer of electroniccoins between two peer-devices. However, it should be noted that thevarious aspects of the invention are not limited to a Digital Cash/eCashcontext, as it will become apparent from the following description.Aspects of the invention include design aspects of a tamper-protectedhardware which is for example usable for transferring electronic tokensamong two peer-devices; strategies and design aspect to improveresistibility of the tamper-protected hardware against attacks (ingeneral), e.g. by means of a hardware PUF, cocoon PUF, etc.; abootstrapping mechanism using special software modules (also referred toas TRUSTLETs herein) to avoid the execution of manipulated or otherparty code; etc. However, the aspects of the invention are not limitedby this non-exhaustive list of aspects. Please note that for all caseswhere certain functionality is provided by some piece of hardwareaccording to one of the various aspects of the invention herein, theinvention also envisions protection for methods and systems making useof such piece of hardware.

Terminology

In the following paragraphs the meaning of some terms, frequentlyoccurring in this document, will be defined before describing thevarious aspects of the invention in further detail.

Electronic Tokens

In this document, the term “electronic token” is used to denote anasset, liability, securitization of debt as a principle of anoriginate-to-distribute model, or secrets that are represented in adigital format, e.g. in form of a digital certificate. With respect tothe some embodiments of related to the off-line transaction method ofelectronic tokens as discussed in the priority applicationPCT/EP2011/001219 by the same applicant, electronic tokens can bedefined as provided in either one of these two PCT applications.

Moreover, it is assumed that the electronic tokens are provided in formof certificates that are issued by an issuing authority. Please notethat there may be multiple issuing authorities of electronic tokens inthe system, and electronic tokens of different issuing authorities maybe the receivable in a transaction. In essence, the asset, liability,securitization of debt as a principle of an originate-to-distributemodel or secret subject to a respective electronic token (andthe—optional—remaining contents of the electronic token), is signed bythe issuing authority.

Each electronic token can be checked and tracked down the chain of trustagainst the root certificate (also called Type-VI certificate herein) ofthe Root Certification Authority (RCA) as provided in the underlyinghierarchical Public Key Infrastructure (see for example IETF RFC 3280,Internet X.509 Public Key Infrastructure, Certificate and CertificateRevocation List (CRL) Profile”, available at http://www.ietf.org orITU-T X.509, “Information technology—Open systems interconnection—TheDirectory: Public-key and attribute certificate frameworks”, 2008available at http://www.itu.int). As it will be outlined below, in oneexemplary embodiment of the invention, the public key part of thesigning key of the Root Certification Authority may be provided immuneagainst counterfeit in the tamper-protected hardware module (e.g. in aROM or EEPROM, “hardcoded” integrated circuit (IC) or other hardwarestructure thereof), so that even in case of attacks against the system,a manipulation can be detected by checking a certificate down the chainof trust to the root certificate. Depending on the application of theinvention, the Root Certification Authority and the issuing authoritymay be or may not the same authority.

Electronic Wallets (e Wallets)

eWallets are electronic devices containing at least one secure moneypurse chip (an exemplary embodiment of the tamper-protectedsemiconductor module, which is sometimes also referred to as CASTOR ornano-safe herein) with integrated and external non-volatile storage.eWallets may be capable of storing and transferring electronic tokens(eCoins) off-line from one eWallet to another eWallet using theteleportation transfer scheme as described in the priority applicationPCT/EP2011/001219 of the same applicant. eWallets in their entiretycould be considered to build an ecosystem für digital-cash-likecurrencies. Their cross-linking accession is based on theIP-infrastructure (IPv4, IPv6). This means a direct one-to-onetransformation of the common knowledge cash-exchanging way from thephysical space into the Internet space without any intermediates. FIG.18 exemplarily shows a stand-alone eWallet according to one exemplaryembodiment of the invention. The eWallet may for example comprise adisplay and a multi-touch-panel or other input means for entering datainto same. Optionally, the eWallet can also be equipped with anaccelerometer as an input means allowing the user to interact with theeWallet. For example, by the user's movement of the eWallet can bemeasured by the accelerometer and could be translated into a user input.The movement of the eWallet by the user may thus for example acknowledgea question to the user, e.g. to authorize a transaction, such as apayment. The eWallet may also have one or more communication-interfaces,e.g. in order to allow for eWallet-to-eWallet transactions andeWallet-to-Internet connections (which can be operated without anyexternal host). Of course, the construction of eWallets is notrestricted to such a complete scenario, due to cost and other reasons itwill be possible to design them to include only functional elementsallowing the eWallet to work within an envisioned host scenario.

Signing and Certificates

A digital cryptographic signature is based on a mathematical scheme todemonstrate and provide the authenticity of a digital message ordocument by a proof. A valid digital signature (digital signatureschemes herein are based upon cryptographic principles) makes arecipient believe the message has been created by a known sender, and ithasn't altered during transition. Digital signatures are commonly usedfor software distribution, financial transactions, and in other caseswhere it is important to detect forgery or tampering. They employ a typeof asymmetric cryptography. For messages sent through a non-securechannel, a properly implemented digital signature gives the receiverreason to believe the message was sent by the claimed sender. Digitalsignatures can also provide non-repudiation, meaning that the signercannot successfully claim they did not sign a message, while alsoclaiming their private key remains secret; further, some non-repudiationschemes offer a time stamp for the digital signature, so that even ifthe private key is exposed, the signature is valid nonetheless.

Digitally signed messages may contain anything representable by abit-string: Examples include keys, electronic tokens, electronic mails,contracts, or messages sent by the presented off-line transactionprotocol. A signature can be applied to a bit-string (representing thedata record/message to be signed) by using a hash function to calculatea hash value of the bit-string and to encrypt the hash value using aprivate signing key of the signing party. The encrypted hash value formsthe so-called signature of the bit-string. Authenticity of thebit-string can be verified by a party by freshly calculating the hashvalue of the bit-string using the same hash function as used in signingthe bit-string, and comparing the freshly generated hash value with thedecrypted hash value of the signature—if the hash values match eachother, the bit-string has not been altered. For decryption of theencrypted hash value (i.e. the signature), the counterpart public key ofthe private signing key of the signing party is used. This public keymay be for example provided by a public key certificate (see below).

In the simplest form, a message and its signature is also called a“certificate”. An electronic token that is implemented as a certificatethus contains at minimum a data record of the electronic token and asignature applied on the data record by the issuing authority. Inanother embodiment, the electronic tokens are provided in form of ITU-TX.509-compliant (version 3) certificates. For example, considering aneCash system, an eCoin can be considered to comprise a data recordcontaining at least the value of the eCoin and a serial number thereof,and optionally currency information. The eCoin is provided in form of acertificate, i.e. contains the data record signed by the issuingauthority.

In cryptography, a public key certificate (also known as a digitalcertificate or identity certificate) is an electronic document whichuses a digital signature to bind a public key with a data record. Inmost cases this data record contains identity information such as thename of a person or an organization, their address, etc. These datarecords conclude not only information about a Root CertificationAuthority (RCA) and Certification Authorities (CAs) based on the ITU-TX.509 (version 3) PKI trust standard, but also about electronic mints(eMints), tamper-protected semiconductor modules (CASTORs/eWallets),electronic tokens (such as for example eCoins), acknowledge tickets,challenges and responses, eDocs and other system-related objects.

The public key certificate can be used to verify that a public keybelongs to an entity (e.g. an individual, an organization or atamper-protected semiconductor module). The digital signature(s) on acertificate are attestations by the certificate signer that the datarecord information and the public key belong together.

Summary of Functionality & Design of the Tamper-Protected Hardware

As noted above, aspects of the invention focus on the functionality ofthe tamper-protected hardware (as for example provided by its firmware),and on the design of the tamper-protected hardware so as to avoid misuseand/or cloning of the tamper-protected hardware and its transactionsystem. It will become apparent from the following that there also oftenis synergy between functionality and design concepts, which, incombination, improve the robustness of the transaction system.Furthermore, it should be noted that the different designs improve thetamper-resistance on the chip level and/or on the semiconductor modulelevel and so far on the device level.

As to the functionality of the tamper-protected hardware, same may beinter alia responsible for executing the off-line transaction protocolaccording to one of the various embodiments described in priorityapplication PCT/EP2011/001219 of the same applicant in atamper-protected environment, which is one important security aspect ofthe system disclosed in the priority application. The respectivefunctionality may be provided by the firmware (e.g. by means ofTRUSTLET(s)—see below) or may be provided in some dedicated hardware ofthe tamper-protected hardware.

A tamper-protected hardware module (as part of a tamper-protectedsemiconductor module) could be for example a tamper-protected die orprovided as a tamper-protected IC. The tamper-protected IC may be forexample implemented as a tamper-protected die or multi-die having atamper-protected enclosure as will be outlined below in more detail.

Furthermore, for enabling communication between the two tamper-protectedsemiconductor modules via a peer-to-peer link, some communicationinterface needs to be accessible and usable by a tamper-protectedsemiconductor module. Depending on the implementation, atamper-protected semiconductor module may be provided with one or morewired and/or wireless communication interfaces directly, or one or morewired and/or wireless communication interfaces may be implementedinternally in the device housing or externally to the device housingaccessible to the tamper-protected semiconductor module via an (internaland/or external) I/O-interface (I/O-port) thereof. For example, thetamper-protected semiconductor module and the communication interface(s)could be provided in a host device, which could be—depending on theapplication—a PDA, PAD, mobile (smart) phone, or other portable device,a cash system provided at a Point of Sale, Access Control System (e.g.in a subway, in an airport, at train state, in an aircraft, fair ground,sports stadium, cinema, etc.), just to name a few.

A device comprising a tamper-protected semiconductor module could thusbe provided with one or more wired and/or wireless interfaces Thewireless interfaces could include at least one of a Radio FrequencyIDentification (RFID) interface, a Near Field Communication interface(NFC), a Bluetooth interface (BTI), a WLAN interface (IEEE 802.11), aWiMAX interface (IEEE 802.16), a WIDI, WirelessHD or WiHD interface, aWireless Gigabit Alliance (WiGig) interface, a Wireless Home DigitalInterface (WHDI), a Wireless USB interface, and a mobile communicationsinterface according to a 3GPP or 3GPP2 standard. The wired interfacescould for example include at least one choice of either an USBinterface, an Ethernet interface (IEEE 802.3), a PanelLink interface, aDisplayPort interface, a SATA interface, a Light Peak interface, aThunderbold interface, a HomePlug interface, a PowerLAN interface, aPowerline Communication (PLC) interface (IEEE 1901) and a FireWireinterface (IEEE 1394).

Generally, one could also consider combining tamper-protectedsemiconductor modules with additional hardware elements, such as memory(volatile memory and/or non-volatile memory), communicationinterface(s), etc. within the same die and/or another enclosure, e.g. aseparate shared housing, which may be referred to as an electronicwallet (eWallet) or as single IC. This electronic wallet may then inturn be integrated into the respective devices.

Another functionality provided by a tamper-protected semiconductormodule according to another embodiment of the invention is thepossibility to perform a secure bootstrapping process of its hardwaremodule, thereby avoiding malicious firmware being executed on thetamper-protected semiconductor module. This concept of performing asecure bootstrapping by means of TRUSTLETs (i.e. specially designedsoftware modules of executable code) is another aspect of the inventionwhich is not limited to use in a tamper-protected semiconductor modulebut may be used for bootstrapping any hardware in general, independentof their utilization in a transaction system.

The PUF Mechanism

A Physical Unclonable Function or PUF is a function that is embodied ina physical structure and is easy to evaluate but hard to predict.Further, an individual PUF device must be easy to make but practicallyimpossible to duplicate, even given the exact manufacturing process thatproduced it. In this respect it is the hardware analogue of a one-wayfunction.

Rather than embodying a single cryptographic key, PUFs can be used toimplement challenge-response authentication. When a physical stimulus(excitation) is applied to the structure, it reacts in an unpredictableway due to the complex interaction of the stimulus with the physicalmicrostructure of the device. This exact microstructure depends onphysical factors introduced during manufacture which are unpredictable.The applied stimulus is called the challenge, and the reaction of thePUF is called the response. A specific challenge and its correspondingresponse together form a Challenge-Response-Pair (CRP). The device'sidentity is established by the properties of the microstructure itself.As this structure is not directly revealed by the challenge-responsemechanism such a device is resistant to spoofing attacks.

Uncloneability means that each PUF device has a unique and unpredictableway of mapping challenges to responses, even if it was manufactured withthe same process as a similar device, and it is infeasible to constructa PUF with the same challenge-response behavior as another given PUFbecause exact control over the manufacturing process is infeasible.

Different sources of physical randomness can be used in PUFs. Adistinction is made between PUFs in which physical randomness isexplicitly introduced and PUFs that use randomness that is intrinsicallypresent in a physical system. To conclude, that means the secrets arenot stored anyway in digital form on the device containing the PUF orelsewhere.

According to another aspect of the invention, hardware modules could beprovided with a hardware structure that provides a Physical UnclonableFunction (PUF) which can aid the improvement of their tamper resistance.A set of known challenges and their respective corresponding responsesof the hardware PUF could then be used to verify integrity of thetamper-protected hardware modules: Basically, the hardware structureimplementing the hardware PUF could be challenged by one or morechallenges for which at least one response to a challenge is known, andin case the tamper-protected hardware module is a “cloned”tamper-protected semiconductor module, the hardware PUF responseprovided by the hardware structure will not match the expected and knownresponse(s).

In another optional implementation the response of the hardware PUF maybe compressed using a cryptographic hash function. For example, inapplications where a response of the hardware PUF is used only to bechecked for being correct (i.e. matching/not matching a known responseof the un-tampered hardware module) this implementation may beadvantageously used to reduce the size of the response data, which couldbe for example of interest in case one or more responses of the hardwarePUF need to be provided via a communications interface to another partyproviding the challenge(s). In this alternative exemplary implementationa response of the hardware PUF to a given challenge can be verified (andthereby the integrity of the tamper-protected hardware module) bycomparing the hash value thereof to the hash value of the known responseof the hardware PUF to this given challenge. Of course, the samecryptographic hash function needs to be used to (re)calculate the hashvalues.

Moreover, in a more advanced implementation, prior to “hash compression”of a response by the hardware PUF, the response is stabilized againstminor deviations introduced by the physical hardware nature of thehardware PUF. Such stabilization could be for example realized byfunctionality used within information and coding theory for applicationsin computer science and telecommunication like error detection andcorrection or error control techniques (Error-Correcting-Code, ECC) usedfor the detection of errors and reconstruction of the original,error-free data.

Another possible way to use the hardware PUF for securing thetamper-protected hardware module is its utilization in a segmentationprocess of secrets used within the tamper-protected semiconductormodule: A secret is split into two parts using a so called “constructorfunction” (one exemplary way to achieve that is one aspect of theVernam-Chiffre by Gilbert Vernam): Such construction function uses thehardware PUF to produce a file called Helper-Data which constitutes oneof the secret's parts. Due to the unique nature of the hardware PUF(fingerprint), the Helper-Data is also unique. The original secret canonly be reconstructed if both, the (correct) hardware PUF (fingerprint)and the correct Helper-Data, are applied. This procedure can be used toavoid digitally storing a secret in the hardware. The secret, once used,can be removed from all internal semiconductor registers and memories.The secret will be extracted from the hardware as required.

Chip-Internal Hardware PUFs

For example, in one exemplary embodiment of the invention, thetamper-protected hardware module comprises a hardware structureproviding a PUF. A processor unit comprised by the tamper-protectedhardware module provides, to the hardware structure implementing thePUF, at least one challenge, for which the correct PUF response byhardware structure implementing the PUF is known. The processor receivesa respective PUF response for each challenge provided to the hardwarestructure implementing the PUF, and verifies integrity of thetamper-protected hardware module by checking whether the response to theat least one challenge for which the correct PUF response is known ismatching the response received from the hardware structure implementingthe PUF.

According to a more advanced embodiment of the invention thetamper-protected hardware module further comprises an I/O-interface forreceiving at least one challenge for which the correct (cryptographichash of) hardware PUF response by hardware structure implementing thehardware PUF is known. The respective (cryptographic hash of) thehardware PUF response for each challenge applied to the hardwarestructure implementing the hardware PUF could be then made availableoutside the tamper-protected hardware module through the I/O-interface.

This would e.g. allow external authorities or devices to check theintegrity of the tamper-protected hardware module. Hence, thechallenge(s) to the hardware PUF could be received from a devicecomprising the tamper-protected hardware module (i.e. from an “externalcomponent” of the device) or on-line from another, external device.Likewise, the respective (cryptographic hash of) hardware PUF responsesfor each challenge provided to the hardware structure implementing thePUF could be provided to the device comprising the tamper-protectedhardware module or on-line to the external device, respectively.

An integrity check of the tamper-protected hardware module utilizing thehardware structure providing the PUF may for example be atamper-protected hardware module function, implemented by the firmware(e.g. upon power-up or reset of the tamper-protected hardware module).For this purpose, in another embodiment, the tamper-protected hardwaremodule may further comprise a storage unit for storing a set ofchallenges and their (known) corresponding (cryptographic hash of)hardware PUF responses. In one exemplary embodiment, the challenges andtheir corresponding (cryptographic hash of) hardware PUF responses aresigned by a Root Certification Authority to allow verification of theirintegrity.

The tamper-protected hardware module may verify the integrity of atleast one challenge and its (cryptographic hash of) hardware PUFresponse based on its signature. The challenges and their corresponding(cryptographic hash of) hardware PUF responses could for example besigned by the Root Certification Authority using a signing key-pair'sprivate signing key, while the public key is provided immune againstcounterfeit in the tamper-protected hardware module, is used to verifythe integrity of the challenges and their corresponding (cryptographichash of) hardware PUF responses.

Moreover, another embodiment of the invention relates to atamper-protected semiconductor module provided with a tamper-protectedhardware module having a hardware structure implementing a PUF, asdiscussed above. Further, another embodiment of the invention provides adevice which comprises such tamper-protected semiconductor module.

The type of the hardware PUF can be of standard hardware (silicon,optical, magnetic, acoustic, coating, butterfly, or intrinsic) PUF, oreven a cocoon PUF.

Semiconductor Modules with Cocoon PUFs

Another aspect of the invention is related to the design of thetamper-protected semiconductor module so as to avoid misuse and cloningof the tamper-protected semiconductor module. Please note that thisaspect is independent of the other aspects of the invention providedherein, but—in some embodiments of the invention—may nevertheless beused in combination with them to further improve tamper resistance.

The basic approach underlying this aspect of the invention is to designa semiconductor module which has unique properties with respect to thedeflection of a physical measurand in response to an excitation (whichcould also be denoted a challenge). These unique deflection propertiesare caused by natural variances or deliberate stochastic parametervariations introduced during the production process of the semiconductormodule and are thus capable of uniquely identifying the semiconductormodule. Further, any attempt to tamper the semiconductor module willalter its deflection properties, which makes tampering attemptsobservable by the semiconductor module. Therefore, it is referred to as“tamper-protected”. Especially in a “cocoon PUF” (as explained below),opening the cocoon in any way will destroy the recreation ability (andthe respective secret).

In one exemplary embodiment of the invention a tamper-protectedsemiconductor module comprising a hardware module is provided. Thishardware module comprises one or more emitters to emit a predeterminedexcitation which can be measured as a physical measurand (German:Messgröße). Further, the hardware module may comprise one or moresensors for sensing the (back-) scatter of the predetermined excitationas physical measurand in a contactless and/or contact-based manner. Thetamper-protected semiconductor module further has a cocoon encasing thehardware module. This cocoon may also be referred to as a package orhousing. The cocoon is configured to influence the (back-) scatter ofthe predetermined excitation to be detected by the sensor(s).

Qualities

PUF systems can be graded based on quality criteria. Exemplary criteriaof relevance are:

-   -   How gets a manipulation (of the PUF itself and on the        surrounding circuitry/circuit) detected?    -   In case of detection, how is the system able to react? Does it        need electrical energy for it?    -   How big is the expected (statistical) failure rate, what happens        if the attack gets not detected?    -   What's the level of protection loss of the whole system in case        of single successful attacks?    -   How big is the amount of information the system is able to store        within the analog PUF?

Even the last criterion is important in times, where today's symmetricalkeys have sizes about 256 bits and digital certificates are even a lotbigger.

As already said, cocoon PUFs jacket the protected system, they are acomponent of a functionalization of the package, extending themechanical physical functionality to provide structural and logical dataintegrity. In case the protected device is an electrical semiconductormodule, the connection terminals (wire connectors to the bond landingsareas/pads of the die) may be passed through the cocoon to realize theouter connection to the Printed Circuit Board (PCB).

FIG. 22 shows a cross-sectional view of tamper-protected chipexemplarily based on a BGA packaging according to an exemplaryembodiment of the invention. As depicted in FIG. 21 the cocoon is not apart of the cocoon PUF circuitry, it is even not part of the die. It canbe considered to form an encasement, which may be connected—in oneexemplary embodiment of the invention—by electrode coupling with thededicated cocoon PUF circuitry in the inside.

The cocoon itself consists for example of a binding matrix (e.g. made ofa synthetic material, ceramic, etc.). This matrix may be formed by thepotting material with less or even no interactions to the usedexcitation principle. This principle implies matter containing smartstructured materials and/or clusters, also referred to as granulesherein. Examples of matter with the required attributes are metallicand/or polymer-based particles/composites in dielectric and/or magneticmatrices. Their attributes based on physical effects like electronicconductivity ranging from single-electron hopping and tunneling topercolation, surface plasmons giving rise to characteristic opticaladsorption, ferromagnetic single domain behavior and superparamagnetism,granular giant magnetoresistance, and enhancement of catalytic activity.A profile of an exemplary utilization is named CASTOR and depicted inFIG. 22.

The packaging in FIG. 22 shows in its center the circuit carrier (namedMulti-layer substrate/Interposer) as the base for the mounted bare-dieor chip (silicon device) connected in this example as flip-chip throughthe use of ball grids or bumps forming the protected CASTOR (theprotected device soldered on the PCB). The covering cocoon builds thesurrounding wrapping consisting of a potting material including aplurality of (in material, size and form) different types of granulatesfrom nano-up to macro-sized particles. All connection terminals (can behundreds and even more) are going through the cocoon to connect theCASTOR chip with the PCB (in the exemplary case via ball grids). Thereis optionally a shielding coat in combination with a finishing housingon the outer side of the cocoon. The mechanism of action of a shieldingis a combination of reflecting and absorbing or attenuating anexcitation, so that only very little excitation would penetrate theshield.

In this example the shield jackets the housing of the cocoon PUF. Inanother exemplary implementation, the potting material with the includedgranules, the shielding and the housing is integrated into a singlecocoon unit.

Utilization

FIG. 21 shows a block diagram of a tamper-protected semiconductor module(nano-Safe) including a cocoon PUF according to an embodiment of theinvention. In the analogue domain, the module comprises a cocoon PUF andan emitter-/sensor phalanx for emitting the excitation of the cocoon PUFand receiving its back-scatter. The emitter-/sensor phalanx can be alsoconsidered part of the digital domain of the tamper-protectedsemiconductor module (nano-Safe) since it may also provideanalogue-to-digital conversion (ADC) and vice-versa (DAC).Alternatively, a separate ADC and/or DAC circuitry could be providedwithin the tamper-protected semiconductor module (nano-Safe). Moreover,the digital domain of the tamper-protected semiconductor module(nano-Safe) comprises a fuzzy-extractor for accessing Helper-Data and akey control unit. The digital domain may also be referred to as thecocoon PUF circuitry. The key control unit may be communicativelycoupled to further circuitry of the tamper-protected semiconductormodule (nano-Safe) that may use the secret of the tamper-protectedsemiconductor module (nano-Safe). This circuitry (“Third ‘IP’ Party'scircuitry using the Secret”) may be part of the tamper-protectedsemiconductor module (nano-Safe).

After the die production (of the semiconductor die, the Q&A selection,the integration into its housing) process, the tamper-protectedsemiconductor module (nano-Safe) receives its secret: Within acontrolled environment (meaning not within the fab, but under control ofthe IP holder), the secret is injected from the outside into the cocoonPUF circuit (e.g. through the key control unit) as depicted in FIG. 21.This procedure can be also called enrollment and is comparable to thepersonalization of smart card systems. During this enrollment the normal(current) state of the cocoon PUF is measured, the resulting fingerprintextracted out of the response of the cocoon PUF is then intermixed withthe injected secret and interwoven within the encoding process for thesecret. This takes care of facts like variations introduced by themanufacturing and the mechanical tolerances of the individual cocoon PUFand is used as part of the reference of future acquired (real-time)measurands.

This mechanism in combination with a so called Helper-Datareconstruction allows a flexible secret regeneration (only from theinside of a cocoon). Such a recombination of two (previously splitwithin the segmentation process) data record parts is comparable to theapplication of one aspect of the Vernam-Chiffre by Gilbert Vernam. Thisallows the implementation of a “Physically Obscured Key” (POK) storagemechanism, mentioning the fact, that the secret gets “distributed” intothe cocoon PUF structure: The fixed fingerprint of the cocoon PUF iscryptographically combined with the changeable Helper-Data to form thesecret. The medium for the Helper-Data is rewritable digital memorystorage, the fingerprint is property of the cocoon. Their combinationreveals the secret.

The packaging of the hardware module (in one exemplary embodiment of theinvention the semiconductor die) mutates into an analog read/writestorage based on the above described mechanism, only capable to read andreconstruct the secret from the inside of the cocoon protectedenvironment.

Principle of Protection

From a hackers perspective, the aim is to corrupt the functionality ofthe protected device by changing the firmware (software used by theprotected device) or hardware (conducting paths/wires, transistors,diodes, gates, memory cells et. al. functionality). This has the need of“tunneling through” the cocoon as a precondition. The creation of this“tunnel” will change the fingerprint of the cocoon PUF in a significantway, so that the previously stored secret (POK) will never again beingable to reconstruct; the secret is vanished. This does not depend on afailed target/actual-comparison, but on the change measurands acquiredfrom the cocoon PUF after the “tunneling through”, which do no longercorrespond to the original ones to needed to reconstruct the originalfingerprint. This is fundamental based on the fact that the cocoon isideally a 360 degree protection implemented by functionalization of thepacking (the cocoon). The use of cocoon PUFs in tamper-protectedsemiconductor modules/chips therefore targets and aims the developmentof a new cryptographic security primitive (element).

Cocoon PUFs based on material effects are not build on or constructedwithin the semiconductor die, but implemented in the cocoon wrappingthereof. This adds a lot of degrees of freedom to the productionproceedings (no limits as introduced by the silicon process) and even tothe Electronic Design Automation (EDA) process of the die. Only therequired analysis electronic (e.g. the fuzzy extractor for carrying outthe fingerprint and the emitter-/sensor phalanx) may thus be situated onthe die. Another positive aspect is the (higher) amount of material incomparison to the protected system (e.g. the die), which may allow for ahigher storage capacity for the secret in comparison to other known PUFsystems.

The idea underlying a resistant wrapping cocoon PUF is to hide secretinformation to be protected in the cocoon structure. In exemplaryembodiments of the invention, a cocoon structure may protect the secretinformation on the molecular level of the cocoon material. Particles(e.g. in different shapes and sizes, including nanosized particles),liquids or gas filled bubbles in a potting matrix effect local changesin permittivity (ε), permeability (μ) or conductivity (σ) following alinear and/or nonlinear unpredictable function. They interact with apredetermined excitation by using the dielectric and magnetic mattermaterial mechanisms of action. This is also referenced in FIG. 19, wherethe common usage of near-field impedance spectroscopy effects ispresented in a consolidated picture.

Their inhomogeneous geometrical distribution dopant density ranging fromthe nano- to macro length scales within the matrix is a stochasticprocess fixed within the production process and guaranties spatialrandom distortions of the measured effects. This makes every cocoonbeing a unique specimen, containing extractable information: The cocoonbecomes an analog storage device (POK).

Since the deflection properties of the inside of the cocoon and thus the(back-) scatter of a given excitation is influenced by natural ordeliberate variations in the production process, e.g. the packagingprocess of the hardware module, the measured (back-) scatter of thepredetermined excitation influenced by the cocoon provides a PhysicalUnclonable Function (PUF) specific to the tamper-protected semiconductormodule. For simplicity the tamper-protected semiconductor module's PUFfunction provided by the unique deflection properties of the cocoon isalso referred to “cocoon PUF”. Using this cocoon PUF, the hardwaremodule—according to another more detailed embodiment of the invention—isable to detect or making impossible any tampering of thetamper-protected semiconductor module, respectively, and non-invasive(or “passive”, e.g. scanning the tamper-protected semiconductor modulethrough a beam of (LASER) light, other radiation fields or screeningtechnology), semi-invasive and full-invasive attacks to access theinside of the cocoon, respectively the hardware module.

Furthermore, in another embodiment of the invention, the hardware modulemakes itself temporarily or permanently unusable, once the hardwaremodule detects a tampering of the tamper-protected semiconductor module.

Please note that in the following it will be often referred to“emitters” and “sensors” in plural. However, also implementations wherethe hardware module has only one single emitter and/or one single sensorare encompassed by this invention.

How the tamper-protected semiconductor module is using its cocoon PUFfunctionality to detect tampering may be implemented in various ways.For example, in one exemplary embodiment of the invention, thetamper-protected semiconductor module may generate a digital “measuredfingerprint” (denoting the fingerprint that has—so to say—just been“measured” by the sensors) of the tamper-protected semiconductor modulefrom the measured values (German: Messwerte) of the (back-) scatter ofthe excitation provided by the respective sensors and detects an attemptto tamper the tamper-protected semiconductor module by comparing themeasured digital fingerprint with a digitized fingerprint of theun-tampered tamper-protected semiconductor module (which may have beenfor example stored previously). It should be apparent that repeatedverification of the integrity of the cocoon, respectively, thetamper-protected semiconductor module requires the same predeterminedexcitation emitted in each verification process, i.e. each measurementof the (back-) scatter of the predetermined excitation (for details onthe “same” predetermined excitation, please see below).

In another exemplary embodiment of the invention, the tamper-protectedsemiconductor module also generates a measured digital fingerprint ofthe tamper-protected semiconductor module from the measured values of(back-) scatter of the excitation provided by the sensors. Further, aprocessor unit of the hardware module could receive (so-called)Helper-Data from storage, and generates a secret based on the measureddigital fingerprint and the Helper-Data (a two-factor secret, asexplained with reference to the Vernam-Chiffre). In case the measureddigital fingerprint is not sufficiently corresponding to the “correct”(previously measured and digitized) fingerprint of the un-tamperedtamper-protected semiconductor module, the generated (recombinedtwo-factor) secret is incorrect, meaning that it is not corresponding tothe “correct” secret that would have been obtained when generating thesecret using the Helper-Data and the “correct” fingerprint of theun-tampered tamper-protected semiconductor module.

In such case, the generation of an incorrect secret implies that theunique deflection properties inside the cocoon must have changed somehow(i.e. the cocoon PUF function of the tamper-protected semiconductormodule has also changed), which in turn indicates an attempt to try toaccess the inside of the cocoon (respectively, tamper-protectedsemiconductor module) yielding a tampering attempt.

There are also different possibilities how to provide one or moreemitters on the hardware module. According to one exemplary embodimentof the invention, the one or more emitters is/are circuitry and/orintegrated circuits of the hardware. Hence, in some applications, theindividual circuit components, wires or some chip areas of the hardwaremodule may have properties of an oscillator/clock generator/radiator andmay thus be used as emitters (thus no separate emitters may need to beused). In order to ensure the emission of the predetermined excitation,the tamper-protected semiconductor module operates the emitters atpredetermined operation conditions during the period in which thesensors are to measure the (back-) scatter of the excitation.

Alternatively, or in addition to those emitter(s) formed by circuitryand/or integrated circuits of the hardware module, in another exemplaryembodiment, the one or more emitters are provided as one or moreoscillators comprised by the hardware modules. The oscillators together(and optionally in combination with the emitter(s) formed by circuitryand/or integrated circuits of the hardware module) emit thepredetermined excitation.

Moreover, it should be noted that the excitation by the emitter(s) maybe an electromagnetic excitation (e.g. electromagnetic fields (e.g. inthe radio spectrum), X-ray, etc.), an optical excitation (e.g. generatedby LED or LASER elements) and acoustic excitation (e.g. infra- orultrasonic), or combinations thereof. Apparently, the sensors that sensethe (back-) scatter of the excitation are electromagnetic sensors,electric field sensors, magnetic field sensors, acoustic sensors,optical sensors or other radiation sensors, depending on the excitationtype. For example, the sensors and/or the emitters provided as part ofthe hardware module may be semiconductor elements and/orMicro-Electro-Mechanical-Systems (MEMS) and/orNano-Electro-Mechanical-Systems (NEMS). MEMS are also known asMicro-Machines (MM) or Micro-System-Technology (MST).

Furthermore, in one further embodiment of the invention, the cocooncomprises a shield to prevent interference with the predeterminedexcitation from the outside of the tamper-protected hardware. Anotheraspect of the shielding is to prevent undesirable emittance of theexcitation to the outer world beyond the limits of the cocoon serving ascovering (e.g. Electro-Magnetic-Interference, EMI, in case theexcitation is of this type). Accordingly, the shielding ensures thesensors measure the (back-) scatter of the predetermined excitationwithin the limits of the cocoon (respectively, the shielding providedtherein). The required type of shielding is of course depending on thetype of the excitation by the emitter(s). Hence, the shielding can be anelectromagnetic shielding, an electric shielding, a magnetic shielding,an acoustic shielding, an optical shielding or a combination thereof.

In a further exemplary embodiment of the invention, the shieldingcomprises or is a metallic plumbiferous or aluminum based shield.

In embodiments of the invention, where an acoustic shielding is desired,the shielding material may for example comprise or consist of structuredpermanently elastic foam like compounds. However, other materials thatabsorb sound and attenuate the acoustic excitation may also be used toprovide an acoustic shield.

According to another embodiment of the invention, an optical shieldingmay comprise one or more surface reflectors or could be implemented as asurface reflector at least a portion of the hardware module. Forexample, in one exemplary implementation, the surface reflector(s) couldbe made of metal.

Further, in another embodiment of the invention, the cocoon isnon-transparent in the optical and infrared spectrum of electromagneticradiation. This feature may for example be advantageous to preventnon-invasive attacks on the inside of the tamper-protected semiconductormodule (respectively, its cocoon as the outer part), e.g. by means ofinfrared light or other optical scanning methods.

Moreover, in another embodiment of the invention, the cocoon may have ahigh thermal conductivity. This may be advantageous in that heat(generated by the operation of the hardware modules circuitry andcircuit components) may be provided to the outside of the cocoon,respectively, cooling of the hardware modules from the outside of thesurrounding cocoon becomes possible.

In order to provide unique deflection properties inside the cocoon, thedesign of the cocoon (besides adding a shield) may ensure for suchproperty. For example, in a further embodiment of the invention, thecocoon comprises a coating and/or a potting compound including amaterial or material mixture (e.g. granulate added to a conventionalcoating/potting material, such as a ceramic or a synthetic material)influencing the (back-) scatter of the predetermined excitation. Pleasenote that the coating and/or potting is applied inside the shielding.

For example, in one exemplary implementation the material or thematerial mixture comprises a granulate whose granules are influencingthe (back-) scatter of the predetermined excitation. In a more detailedimplementation the granules of the material or the material mixturecould for example have different mechanical dimensions: They may be ofdifferent size (e.g. nano-sized granules to macro-sized granules) and/ordifferent shape.

The granules of the material or the material mixture may benon-regularly (randomly) distributed in the potting material therebyinfluencing the (back-) scatter of the predetermined excitation in wayunique to the tamper-protected semiconductor module. This may be forexample achieved by the production process that is providing the coatingand/or potting to form the cocoon encapsulating the hardware module. Inone example, the potting material is made of ceramics and may contain agranulate of the material or the material mixture influencing the(back-) scatter of the predetermined excitation.

In another exemplary embodiment of the invention, the material ormaterial of the shielding or the granules thereof are similar to thematerial mixture of the hardware module and/or its bonding with respectto their chemical characteristics (and optionally the physicalcharacteristics). The idea behind this is to make full-invasive attackson the tamper-protected semiconductor module more difficult: Forexample, the package being or including the cocoon of thetamper-protected semiconductor module might be removed using acids orchemical bases in order to access the inside hardware module. Bychoosing the chemical character of the cocoon's material and thehardware module's material (and/or its bonding) to have similar chemicalresistance against acids/chemical bases, it is difficult to control theetching to remove the cocoon material only without damaging the hardwaremodule or its bonding. In embodiments of the invention, bonding can beunderstood mean “connection” and applies insofar also to otherconnection techniques, e.g. so-called ball grid connections.

Similarity of the physical character may be another optionally desiredproperty, since the physical characteristics influences the reactiontime with the acid/base so that it becomes even more difficult tocontrol the edging to only remove the cocoon material, while notdamaging the hardware module or its bonding.

Regarding the predetermined excitation, there are also various ways togenerate such predetermined excitation. Of course, to be able toreproduce the “fingerprint” (for the un-tampered case) the predeterminedexcitation should be the same for each measuring process that determinesthe actual “just measured” fingerprint. In practice it may be difficultto achieve identical excitations (for example, due to temperaturevariations, drift effects of physical material properties in time, noisymeasurement, fuzzy sensors, etc.). Hence, the same excitations shouldnot be interpreted as 100% identical excitations; however, thevariations may be compensated by the generation process of thefingerprint from the measured values provided by the sensors, and/orcountermeasures may keep variations of the excitation in an acceptablerange not critical to the uniqueness and stability of the generatedfingerprint. In one exemplary embodiment, the cocoon's influence on the(back-) scatter of the excitation to be detected by the sensors isinsensible to change of temperature (in a definable range or desirablerange of temperatures).

Please note, there may be a tradeoff, respectively a mutual influence,between the foreseeing countermeasures keeping variations of theexcitation in an acceptable range not critical to the uniqueness andstability of the generated fingerprint and the generation process of thefingerprint from the measured values provided by the sensors (i.e. themeasured values so to speak form the data base for the determination ofthe fingerprint): On the one hand, the generation process of thefingerprint may not reduce the data spectrum for the generation of thefingerprint in a way it loses its uniqueness; on the other hand,countermeasures keeping variations of the excitation in an acceptablerange may aid to minimize the reduction of the data spectrum for thefingerprint but may be expensive to implement.

To generate a sufficiently wide data spectrum i.e. a sufficient numberof measured values (respectively “bits” once the measured values arequantized) that can be used for the generation of the unique fingerprintof the tamper-protected semiconductor module, different possibilities todesign the predetermined excitation of the one or more emitters arepossible. For example, one possibility may be that the predeterminedexcitation is a constant predetermined excitation, i.e. not varying overtime. Alternatively, the predetermined excitation could also be anexcitation varying in a predetermined manner—which implies the sensorsshould measure the measurand multiple times within a measurement period.The variation of the excitation could for example be implemented by thehardware module comprising an excitation controller that controls theemitters individually or in combination to produce the predeterminedexcitation varying in time. For example, different sets of emitters areused in a given sequence in time to generate respective excitations(i.e. a predetermined sequence of individual excitations) that form thepredetermined excitation.

In a further embodiment, the sensors to provide a measured value of themeasurand that is to be part of the data base for generating thefingerprint could be controlled by the hardware module. For example, thehardware module may trigger/query different sets of sensors at differenttimes of a measurement cycle to receive their measurement results. Thiscontrol of the sensors may of course be readily combined with thecontrol of the emitters described in the preceding paragraph.

Moreover, the hardware module protected by the cocoon may virtually beany hardware structure that should be made tamper resistant. In oneexemplary embodiment of the invention, the hardware module is a die, amulti-die, or a chip comprising of one or more integrated circuits(ICs). Likewise and in another embodiment of the invention, the hardwaremodule is a die, a multi-die, or a chip comprising of one or more ICsassembled on a circuit carrier (e.g. by means of a bonding process).

As indicated above, it may be possible that circuitry and/or integratedcircuits of the hardware module is/are used as the one or moreemitter(s) of the predetermined excitation.

With respect to the electromagnetic signal exchange nature, it is alsopossible to “check the channel” before executing the reconstructionprocess revealing the secret. Such a Physical Carrier Sensing (PCS) ofthe environment through passive scanning for later collision avoidanceattitude may be useful to optimize the procedure.

In order to allow the emission of a predetermined excitation, accordingto one embodiment of the invention, the tamper-protected semiconductormodule deactivates the (integrated) circuit parts of the hardware moduleoccasionally during a measurement cycle in which the one or moresensor(s) detect the (back-) scatter of the predetermined excitation.For instance, the deactivation may be a deep power down to minimize theinterference of the (integrated) circuit parts of the hardware modulewith the (back-) scatter of the predetermined excitation.

For the purpose of controlling emitters and sensors and optionally fordeactivating the (integrated) circuit parts of the hardware moduleduring a measurement cycle a measurement unit may be provided on thehardware module. This management unit may be considered an IC of thehardware module. The management unit is provided with an I/O-interfacefor transmitting and/or receiving standardized commands and/or datapackets from the other integrated circuit parts of the hardware module,e.g. the sensors and emitters. For example, the measurement unit maycoordinate the one or more sensor(s) and the one or more emitter(s) in ameasurement cycle.

Please note the cocoon may be the housing of the hardware module or maybe itself included in housing together with the hardware module.Likewise, the shielding of the tamper-protected semiconductor modulecould be part of the cocoon or may enclose same. In case a “separate”housing is provided, the cocoon could be provided in form of a malleablemass between the housing and the hardware module.

Another aspect of the invention is the provision of a method formanufacturing a tamper-protected semiconductor module with a cocoon PUFas described herein. According to an embodiment of the invention amanufacturing method of a tamper-protected semiconductor module with acocoon PUF comprises of providing a hardware module equipped with one ormore emitters for emitting a predetermined excitation that can bemeasured as a physical measurand, and one or more sensors for sensingthe (back-) scatter of the excitation in form of the physical measurandin a contactless and/or contact-based manner. Further according to thismanufacturing method, the hardware module is housed in a cocoon toobtain the tamper-protected semiconductor module, wherein the cocooninfluences the (back-) scatter of the excitation detected by the sensorsin a way unique to the tamper-protected semiconductor module.

In an exemplary embodiment of the invention, housing the hardware modulein the cocoon includes the enclosure of the hardware module in a pottingmaterial, and encapsulating the hardware module enclosed by the pottingmaterial inside a shield to prevent interference with the predeterminedexcitation from the outside of the tamper-protected semiconductormodule. As noted above, the shielding may be but is not necessarily partof the cocoon.

Furthermore, in another embodiment of the invention, the exemplifiedshielding is produced using one or more of the following technicalprocesses for producing different layers of shielding material on theinside and/or outside of the cocoon surface:

-   -   sintering one or more shielding materials,    -   sputtering one or more shielding materials,    -   spraying, spritzing or bubbling one or more shielding materials        (e.g. using High-Volume-/Low-Pressure plants (HVLP)),    -   coating and/or plating one or more shielding materials (e.g.        electric conductive (finishing) paint/lacquers containing/made        of epoxy, polyurethane or acrylic with expletive particles like        silver, silver plated copper, pure copper or nickel),    -   immersing in one or more baths of fluids comprising the        shielding material,    -   evaporating, optional in a vacuum, of one or more shielding        materials (e.g. Physical Vapor Deposition (PVD)),    -   electroless plating one or more shielding materials (e.g. one        layer/sheet/lamina e.g. of pure copper, followed by another        layer/sheet/lamina e.g. of nickel-phosphorus alloy),    -   electrolysis using one or more same shielding materials.        Optionally, the shielding may be subject to (additional) thermal        treatment.

In one exemplary embodiment of the invention, the potting materialcomprises conductive material or a conductive material mixtureinfluencing the (back-) scatter of the excitation to be detected by thesensors provided on the hardware module. Optionally, the manufacturingmethod provides a passivation (e.g. electrical isolation) to thehardware module and/or to its outside.

In a further exemplary embodiment, the provision of the hardware moduleas part of the manufacturing process includes implementing the hardwaremodule in a standard wafer process. For example, the standard waferprocess is a silicon semiconductor process.

A further aspect of the invention is to provide a method to detecttamper attempts against a tamper-protected semiconductor module equippedwith a cocoon PUF (the hardware module of the tamper-protectedsemiconductor module with the cocoon PUF):

-   a) causes the one or more emitters of the hardware module to emit    predetermined excitation, and further-   b) causes the one or more sensors of the hardware module to sense    the (back-) scatter of the predetermined excitation reflected by the    cocoon of the tamper-protected semiconductor module. Further, the    hardware module then-   c) generates a digital “measured fingerprint” from (back-) scatter    of the predetermined excitation sensed by the one or more sensors,    and-   d) verifies the integrity of the cocoon by using the measured    digital fingerprint recorded.

As before, the term “measured fingerprint” is used to denote thefingerprint derived from the just/last measured values of the (back-)scatter of the predetermined excitation sensed by the one or moresensors.

Please note that the hardware module may perform these steps a) to d)not only once, but also repeatedly. Hence, an ongoing verification ofthe integrity of the tamper-protected semiconductor module is possible.Steps a) to d) may be for example repeatedly performed in irregularintervals or on a random basis.

The method according to another embodiment of the invention furthercomprises a step

-   e) of producing an interrupt, in case integrity could not be    verified in step d). This interrupt may cause the hardware module    disable itself permanently or temporarily.    In a further embodiment of the invention, the method further    comprises a step-   f) of deactivating integrated circuit parts of the hardware module    during a measurement cycle in which the one or more sensors detect    the (back-) scatter of the predetermined excitation. This may be    helpful to produce the same predetermined excitation in all    measurements.

Furthermore, as noted previously, there may be different possibilitieshow to make use of the cocoon PUF for verifying the integrity of thecocoon/tamper-protected hardware module. In one exemplary embodiment ofthe invention the integrity of the cocoon is verified by comparing themeasured digital fingerprint (i.e. the digital fingerprint justgenerated from the latest measured values of the sensors) with a digitalfingerprint of the un-tampered cocoon (this latter fingerprint couldalso be referred to as the “reference fingerprint” or “un-tamperedfingerprint). For instance, the fingerprint of the un-tampered cocoon isstored in form of a certificate in a memory.

In a further exemplary embodiment, the method for checking the integrityof the cocoon may further comprise the following steps of

-   g) initializing the hardware module by creating and recording a    fingerprint of the un-tampered semiconductor module,-   h) securing this fingerprint of step g) using a certificate signed    by an Certification Authority, and-   i) storing the certified fingerprint in a semiconductor non-volatile    memory. In one exemplary embodiment, this certificate may be signed    by the Root Certification Authority (RCA) or another Certification    Authority that can be validated against the RCA's certificate    containing the public key part of the RCA's public key-pair.

Another possibility for verifying the integrity of the cocoon is the useof the cocoon PUF as part of a key restoration process (two-factorsecret). Using the cocoon PUF this key restoration process is used torecreate the key (also referred to as “recombined secret”) fromHelper-Data. The Helper-Data is generated based on the key (fingerprint)using the cocoon PUF of the un-tampered cocoon, respectively,un-tampered (tamper-protected) semiconductor module, which binds therecreation of the key (fingerprint) to this specific cocoon PUF. If thecocoon PUF is altered due to a tampering attempt, the key (fingerprint)may be temporarily or permanently destroyed, since the PUF of the cocoonwill be temporarily or permanently changed as a result of the tamperingattempt, so that the key (fingerprint) generation function is unable toreconstruct the correct key (fingerprint) (again) or due to theintention of the designer/IP owner to trigger a backlash. Hence, in thisexemplary embodiment, the verification of the integrity of the cocoon isrealized by means of a key (fingerprint) restoration process generatinga key based on Helper-Data and the detected (back-) scatter of thepredetermined excitation.

The generation of the Helper-Data may be e.g. part of an initializationprocedure of the semiconductor module in an un-tampered case (e.g. byperforming the initialization at a trusted party; a trusted party couldbe for example a Root Certification Authority, the manufacturer of thetamper-protected semiconductor module, the manufacturer of a devicecomprising the tamper-protected semiconductor module, the customer/user,etc.). Therefore, in another embodiment of the invention, the methodfurther comprises the steps of g) initializing the hardware module bycreating and recording a fingerprint of the un-tampered semiconductormodule, and generating the Helper-Data based on the key and the sensed(back-) scatter of the predetermined excitation. This Helper-Data maythen be stored in a non-volatile memory.

Please note that the Helper-Data may not need to be protected, since itis impossible to recreate the secret from the Helper-Data without thecorrect PUF (fingerprint). Hence, the Helper-Data may be stored eitherin the hardware module or in an external storage of a device or anotherhardware structure that contains the tamper-protected semiconductormodule.

Such a cocoon PUF method could be for example made available within thevalue chain on two positions:

-   -   IP: within the EDA tool set as a primitive library element (the        cocoon PUF circuitry without the cocoon itself) in form of a        soft- or hard macro ready for the ASIC designer to “point and        shoot”, and    -   Hardware: within production processes for the chip (fab)        industry as a small change at the packaging point of the        production line.        Encapsulation Through the Cocoon

Cocoon PUFs can be fundamental primitive security vaults well suited forsemiconductor circuitry: A logical similar approach following theconcept of a bank vault, however built mainly from functional(nano)sized material instead of sheathed steel and reinforced concrete.If such a security vault integrates also the necessary capability andintelligence to ‘extract the secret out of the cocoon’ only for theauthorized user-circuitry inside the cocoon (such a behavior is known asread-proof-hardware, where the attacker cannot read information on themedium stored), it is capable to act in and enforce the securityinterests of the designers/IP-owners/users using a flexible policycontrol approach. This is a way to put security at work autonomously andself-reliant in unfriendly environments. Such a system is considered asgeneral suitable to protect cryptographic artifacts (short: secrets)like keys for AES (Advanced Encryption Standard), ECC (Elliptic CurveCryptography), or HASHes (a hash function is any algorithm that maps amostly larger data set to a smaller (fixed length) data set, called thehash (key)), digital signatures in certificates, biometric data (e.g.human fingerprints), customer data (e.g. bank accounts), specialalgorithms or business logic coded in firmware which has to be executedin a secured and controlled environment and other IP based arbitrarydata. This may all come in combination with the ability to providestrong authentication on an open-source Kerckhoffs' principle(cryptosystems should be secure even if everything about the system,except the key, is public knowledge). Moreover, a cocoon PUF is expectedto provide significant more memory capacity than other PUFs, due to thegreater volume of functional material without using space on the siliconitsself (of cource, the measurement logic (e.g. Fuzzy Extractor, KeyControl, emitter-/receveiver phalanx, etc.) is a small fixed part of thesilicon).

Chip designs using a cocoon PUF can help to enhance the attackresistance of System-on-Chip (SoC) devices with local intelligence andprotected firmware (which can be placed inside or outside the securityperimeter), complex CPUs, FPGAs, RFID-chips, containers for cryptomaterial like Secure Element's (SE's) in SIM, USIM, ISIM or SD-Cards,user related biometric information in the new identity card (nPA) andnew passport (ePass) in Germany or wallet chips for digital cash to namea few by releasing them only in a safe-toughened appearance. ASICdesigner do not need to be cryptographic specialists, knowing how to“make chips” will be enough. The silicon part of the cocoon PUF can bemade available as library element so that it can be easily picked andintegrated into the chip-under-development within the standard designElectronic Design Automation (EDA) process.

Hackers Perspective on Tamper Protection

The truth about tamper protection is that it is impossible to hardenintegrated circuits totally against all forms of attacks; this mightalso become true for the cocoon PUF concept. It is assumed, that—withenough ‘energy’ it will always be possible to analyze the functionallogic out of any design. The only and single item which can be defendedup to a sensible limit (security based on tamper-resistance is a matterof economics: the costs in money and time for breaking the system mustbe reasonably higher than the value of the protected assets) is thesecret protected by the cocoon PUF approach. So the strategy of thecocoon PUF concept is to protect the secret and not the device. Thechallenge is to destroy this secret before the attacker can get it. Itis this most important ‘feature’ of being tamper-evident, meaning thatif the (sub-) micron structure is physically damaged (e.g. through anattack), its CRP (Challenge Response Pair) behavior changes noticeablyand hence allows to detect the tampering. The issue at stake here isthat every (semi)-invasive tampering meaning any physical manipulationcauses this and that is it practically impossible to find a way tosnatch the secret. In most PUF implementations, this property is onlyunproven implicit.

If the attacker surmounts the PUF in a way that the secret is stillextractable than the remaining steps to be taken are easy: Tapping theinternal interface to the PUF interface and initiating a secretreconstruction is enough. To prevent such a scenario, the most important“feature” of the cocoon PUF will be its fundamental ability to apply theread-proof effect even in a passive “out of service” supply voltage lessstate and making this inevitable: The fact of a cocoon cover fracture(this is a must to penetrate the cocoon interior “accessing theprotected device” in a direct manner) is a sufficient condition for thesecret to vanish.

Advantages of the Cocoon PUF

Concluding the introduction of the presented cocoon PUF technology, thefollowing advantages may be obtained:

-   -   The cocoon PUF mechanism is based on material-inherent        (non-silicon) functionality,    -   It implements copy protection by making a system uncloneable,    -   It implements misusage protection by making a system insensibly        to reverse engineering,    -   It encompasses passive destructing (without electricity) of the        secret as countermeasure,    -   It operates in potential unfriendly environments under        “paranoidal assumptions” of continuous exposures to threats,    -   It implements self-reliant autarky in combination with autonomy,    -   It needs only a moderate silicon-sized footprint for the        measurement circuitry,    -   It is assumed of being cheap to manufacture on existing        production lines,    -   The application implements and emphasizes ease-of-use, no need        to be a security specialist,        Maze of Knossos—Configuration of the FPGA

Another aspect of the invention is to provide a hardware module (chip)with a FPGA having a configuration of an array of Processing-Elements toprovide a given functionality. Moreover, a storage unit of thetamper-protected semiconductor module could store signed configurationinformation (“fingerprint”) indicating the initial configuration of thearray of Processing-Elements of the FPGA of the hardware module. Thehardware module's processor unit could then verify integrity of thehardware module by verifying the integrity of initial configurationinformation based on its signature and comparing the verified initialconfiguration information with configuration information of theProcessing-Elements of the FPGA. If a mismatch is detected, the hardwaremodule may for example disable itself temporarily or permanently. Asthis mechanism allows detecting attacks on the hardware module it can beconsidered tamper-protected. The principle of a configurationfingerprint may also be implemented with other programmable ICs(hardware structures) that have a unique, programming dependentconfiguration (excluding ROMs). For simplicity, it is referred to FPGAsherein, but the concept outlined herein may also be used in otherprogrammable ICs (programmable hardware modules).

Please note that the use of a hardware structure providing a hardwarePUF for which a set of challenges and corresponding (cryptographic hashof) responses is known and the use of a FPGA whose initialProcessing-Elements' configuration is known within a hardware module inprinciple are two independent aspects of the invention to improve therobustness of a tamper-protected hardware module which may be usedindependently. However, it may of course be advantageous to use thecombination of both of them (and optionally even more protectionsmechanisms) to improve the tamper-resistibility of the tamper-protectedhardware module. In case the tamper-protected hardware module cannotverify its integrity, the tamper-protected hardware module is capable todisable itself temporarily or permanently.

On-Chip Trusted Time Source

Another aspect of the invention facilitating an improvement of thetamper-resistibility of the tamper-protected hardware module is theprovision of an on-chip trusted time-source within the tamper-protectedhardware module. Please note a trustable time-source is required forexample in order to be able to verify the validity of all certificates(which do typically have an expiration date and time). In one exemplaryembodiment, the on-chip of the tamper-protected hardware module'strusted time-source is implemented as a (strong) monotone timer which iserased after a predetermined time span upon disconnection from its powersupply. Please note that the expression “predetermined time span” isintended to express the (strong) monotone timer will be erased once itscounter value cannot longer be held in memory due to disconnection fromthe power supply. Please note also, in case the trusted time source isto be reinitialized after erasure, the tamper-protected hardware modulemay receive on-line the current date and time from a trustableauthority, e.g. a Root Certification Authority or a CA, or from an eMintsigned by a Root Certification Authority, and initialize the trustedtime source with the current date and time only if the signature of theRoot Certification Authority can be verified successfully.

It should be apparent that without a local valid trusted time-source,none of the certificates can be verified, so the reestablishment of thevalidity of the trusted time source should be performed first before anyother protocol communication operation, including any transaction, canbe done.

Crypto-Initialization of the Tamper-Protected Hardware Module (CASTOR)

Another aspect of the invention related to increasing thetamper-resistibility of a tamper-protected hardware module is to providea validation process of the tamper-protected hardware module, whichcould be considered an “initialization” of the tamper-protected hardwaremodule. This validation process/initialization of the tamper-protectedhardware module may be performed at the Root Certification Authority andprovides the tamper-protected hardware module with a certificate used asthe “identity” or identifying descriptor item (e.g. a serial number) ofthe tamper-protected hardware module in any transaction performed. Anytamper-protected hardware module that does not undergo the validationprocess/initialization at the Root Certification Authority and that'sthus not equipped with such certificate will be unusable in thetransaction system. Hence, tamper-protected hardware modules embezzledfrom the production process become unusable.

For this purpose, according to an exemplary embodiment of the invention,a tamper-protected hardware module comprises a key generation unit whichautonomously generates a predetermined set of one or more symmetric keysand one or more asymmetric (public) key-pairs in response to aninitialization request. Optionally, the key generation unit could beadapted to autonomously generate the keys only once upon initialization.

One of these generated keys is used to identify the tamper-protectedhardware module, respectively the tamper-protected semiconductor modulecomprising same, similar to an identifying descriptor item of thetamper-protected hardware. Furthermore, the tamper-protected hardwaremodule is equipped with an I/O-interface for providing the key used foridentification of the tamper-protected hardware module to the outside ofthe tamper-protected hardware module in response to the initializationrequest. Furthermore, through its I/O-interface the tamper-protectedhardware module receives a certificate from a Root CertificationAuthority comprising the key used to identify the tamper-protectedhardware module and stores the certificate comprising the key used foridentification of the tamper-protected hardware module and (togetherwith) the generated predetermined set of symmetric keys and asymmetrickey-pairs in a storage unit.

The certificate of the Root Certification Authority provided as part ofthe initialization of the tamper-protected hardware module may forexample further comprise public keys of the Root Certification Authorityfor encryption and verifying the Root Certification Authority'ssignatures. Moreover, the Root Certification Authority's certificatecould optionally comprise the public key of the public key-pair forinternal encryption and hashing purposes generated by thetamper-protected hardware module in response to an initializationrequest. For example the public keys may be used in the protection of asymmetric key for on-the-fly encryption/decryption of data as to beexplained below (umbrella protection).

In another embodiment of the invention, besides the key/keys used toidentify the tamper-protected hardware module, the key generation unitmay further generate a symmetric key for on-the-flyencryption/decryption (also called umbrella encryption/decryption) ofdata to be maintained and/or stored inside or outside thetamper-protected hardware module in a host device including thetamper-protected hardware module prior to its exchange through theI/O-interface.

Furthermore, the symmetric key for on-the-fly encryption/decryption ofdata could be then stored by or in the external storage of a host devicecomprising the tamper-protected hardware module. In a more detailedexemplary embodiment, the symmetric key is processed as follows prior toits storage:

-   a) The symmetric key is first encrypted using a public key of an    asymmetric key-pair of the Root Certification Authority.-   b) Then the encrypted symmetric key of step a) is encrypted through    a private key of a key-pair used for internal encryption, and    hashing purposes being generated by the tamper-protected hardware    module in response to an initialization request.-   c) Finally, the double-encrypted symmetric key of step b) is signed    using a public key of an asymmetric key-pair of the Root    Certification Authority.

The signed and double-encrypted symmetric key of step c) is then storedby in a host device's external storage comprising the tamper-protectedhardware module (i.e. a storage residing outside the tamper-protectedhardware module).

In one embodiment of the invention, the predetermined set of one or moresymmetric keys and one or more asymmetric key-pairs generated by the keygeneration unit comprises a public key-pair for internal encryption andhashing purposes being generated by the tamper-protected hardware modulein response to an initialization request (i.e. the public key-pair usedin step b) is inter alia used for encrypting the symmetric key foron-the-fly encryption/decryption of data mentioned above).

Moreover, in a more detailed exemplary implementation, thetamper-protected hardware module could on demand provide the public keypart of the public key-pair for internal encryption and hashing purposesto the exterior of the tamper-protected hardware module via theI/O-interface.

For example, the symmetric key for on-the-fly encryption/decryption ofdata could be used to encrypt electronic tokens that then could bestored in encrypted form outside the tamper-protected hardware module ina host device including the tamper-protected hardware module. In casethe tamper-protected hardware module is destroyed or rendered unusable(permanently or temporarily), the encrypted electronic tokens stored ina host device's storage outside to the tamper-protected hardware couldbe recovered by the Root Certification Authority (or alternatively bythe Escalation Authority, EA), assuming that the Root CertificationAuthority maintains the respective tamper-protected hardware's publickey-part of the public key-pair for internal encryption and hashingpurposes, as described above.

The decision to enable or disable this functionality may be up to theuser/owner of the tamper-protected hardware module. Accordingly, in oneembodiment of the invention, the user/owner of the tamper-protectedhardware module can configure whether or not the tamper-protectedhardware module stores the symmetric key for on-the-flyencryption/decryption outside the tamper-protected hardware module. Thesymmetric key for on-the-fly encryption/decryption may be protected bysteps a), b) and c) as mentioned before, before storing same outside thetamper-protected hardware module.

SUMMARY

A tamper-protected semiconductor module may be used as a so calledKey-Wrap-Module (KWM), also referred as Key-Generating-Key (KGK)container, where a KWM cryptographically wraps all existing (a)symmetrickeys, so that they may be safely stored. The KWM initialization processcan be used to transform the KGK combined with an external or internalSW-supplied Initialization Vector (IV). Another usage of a KWM is theproduction of safe master Key-Encrypting-Key (KEK) schemes.

It should become apparent from the foregoing explanations there areseveral individual functional and design-related measures of thetamper-protected semiconductor module making same less vulnerable tomisuse or cloning. It should be apparent that the individual functionaland design measures of the tamper-protected hardware (including alsoSecure Bootstrapping explained below) may be advantageously combined toobtain an overall security architecture that allows achieving a veryhigh-level of tamper resistance of the tamper-protected semiconductormodule.

Summary of Trusted Boot Through TRUSTLETs

As indicated above, another aspect of the invention is the suggestion ofa bootstrapping mechanism for hardware. This bootstrapping mechanism maybe used for bootstrapping a tamper-protected hardware module as used ina transaction system, but could be—in general—also used to bootstrap anytype of IC (firmware) or device (firmware and/or software). Thebootstrapping mechanism is based on the provisioning offirmware/software in form of one or more software module(s) (also calledTRUSTLET(s) herein) each of which providing a piece of the overallfunctionality of the firmware/software—of course, also one (standard)software module could provide the entire firmware/softwarefunctionality. The idea of the bootstrapping mechanism is to generate achain of trust by one software module checking the validity of the nextsoftware module to be executed prior to its execution. If theverification of the subsequent software module fails, the bootstrappingis stopped, which may leave the bootstrapped hardware (temporarily)dysfunctional. The individual software modules are encrypted and signedthemselves which is intended to protect the software modules againstbeing tampered by attackers.

According to an exemplary embodiment of the invention, the softwaremodules are generated as follows: It is assumed that an entity block isprovided to be included into the software module. The entity blockprovides an executable piece of software implementing a given (set of)functionality of the firmware/software. First, the software module issigned: A fingerprint of the (uncompressed) software module is generatedusing a hash function (e.g. by using the Secure Hash Algorithm Standard(SHA—see FIPS PUB 180-3, “Secure Hash Signature Standard”, available athttp://csrc.nist.gov) in order to allow verification of the integrity ofthe software module. This fingerprint is then encrypted using theprivate key of a public key-pair (signing key) to thereby generate adigital signature of the software module.

Optionally, the software module is additionally compressed to reduce itssize. The—compressed or uncompressed—software module is then combinedwith the encrypted fingerprint to form an integrity-protected softwaremodule. This integrity protected software module is encrypted using arandom secret key to thereby form an encrypted and integrity protectedsoftware module.

The random secret key is encrypted using the private key of a publickey-pair, and the software module is obtained by combining the encryptedand integrity protected entity block, and the encrypted random secretkey. Please note that a software module generated according to thisprocess (or according to a more advanced exemplary implementationthereof) is also referred to as a TRUSTLET herein.

Please note, the public key part of a public key-pair used forencrypting the random secret key is assumed to be known to thebootstrapping hardware (in order of being able to decrypt, verify andexecute).

The public key encryption may for example be realized according to oneof the Elliptic Curve Cryptography (ECC) Standards (ECC is included innumerous standards around the world. This issue of Code and Cipherfocuses on three key standards bodies that incorporate ECC: ANSI, IETFand NIST. Standards are: ANS X9.62, WAP WTLS, ANS X9.63, and IEEE 1363,and recommendation NIST SP 800-56). For example, the fingerprint couldbe encrypted with the private key of the key-pair using a DigitalSignature Algorithm (DSA), or an Elliptic Curve Digital SignatureAlgorithm (ECDSA), DSA, and ECDSA can be found within FIPS PUB 186-3,pp. 19 to 26, available at http://csrc.nist.gov.

In one exemplary further implementation, the integrity protected entityblock is encrypted using symmetric-key encryption, such as for examplethe Advanced Encryption Standard (AES, see FIPS PUB 197, available athttp://csrc.nist.gov) or Data Encryption Standard (DES, see FIPS PUB46-3, available at http://csrc.nist.gov). Accordingly, the random secretkey may be a symmetric key.

In order to implement a chain of trust between the software modules uponbootstrapping, the entity block of a software module may compriseinstructions to execute another software module. Basically, theseinstructions may enable:

-   -   decryption of an encrypted random secret key of the other        software module, decryption of the encrypted and integrity        protected entity block of the other software module using the        decrypted random secret key to thereby obtain an integrity        protected entity block of the other software module,    -   decryption of an encrypted fingerprint (i.e. the digital        signature) comprised in the integrity protected entity block of        the other software module, and    -   generation of another fingerprint of the entity block using the        same hash function as used to generate the encrypted fingerprint        of the other software module,    -   verification of the entity block's integrity of the other        software module's integrity protected entity block by comparing        the decrypted fingerprint with the other generated fingerprint,        and    -   execution of the other software module's entity block, only in        case the integrity of the other software module's entity block        is verified successfully.

In a further exemplary embodiment of the invention, the private key usedto encrypt the fingerprint differs from the private key used forencrypting the random secret key. Further, the public keys correspondingto the private keys to encrypt the fingerprint and random secret key,respectively, are assumed to be provided in form of a certificateoriginating from the Root Certification Authority. This means the publickeys are signed by a Root Certification Authority using a privatesigning key (part) of a signing key-pair, the public key (part) of whichis provided immune against counterfeit in the hardware to execute thesoftware module.

Being provided with such software modules (TRUSTLETs), according to afurther embodiment of the invention, a piece of hardware is bootstrappedinto a controlled environment by executing a set of software modules ina given order as follows:

-   a) decrypting an encrypted random secret key of one of said set of    software modules to be executed next,-   b) decrypting an encrypted and integrity protected software module's    entity block to be executed next, using the decrypted random secret    key, to thereby obtain an integrity protected software module's    entity block to be executed,-   c) decrypting an encrypted fingerprint comprised in the integrity    protected software module's entity block to be executed next, and-   d) generating another entity block's fingerprint using the same hash    function as used to generate the encrypted fingerprint,-   e) verifying the entity block's integrity of the software module's    integrity protected entity block to be executed next by comparing    the fingerprint decrypted in step c) with the fingerprint generated    in step d), and-   f) only in case the integrity of the software module's entity block    to be executed next is successfully verified, this software module's    entity block will get executed next, Steps a) to f) are to be    performed for each software module of the set of software modules    thereby realizing a chain of trust.

In one further exemplary embodiment of the invention, the functionalityto perform steps a) to f) for the set of software modules' firstsoftware module is implemented in the hardware (e.g. is stored in a ROM,or the like within the hardware as a gate-based hardcoded (never to bechanged) implementation). The hardware's or its piece(s) providingfunctionality to perform steps a) to f) therefore should beadvantageously tamper-protected.

Furthermore, the processing of a software module to be executed next mayfurther comprise

-   g) obtaining a certificate comprising one or more public keys,    wherein the one or more public keys are signed by a Root    Certification Authority using a signing key-pair's private signing    key (part), the public key (part) of which is provided immune    against counterfeit in the host device, and-   h) verifying the one or more public key(s) (part's) integrity    comprised in the certificate using the public key (part) provided    immune against counterfeit in the host device.

One verified public key of the certificate is used for decrypting theencrypted random secret key in step a) and one verified public key ofthe certificate is used for decrypting the encrypted fingerprint in stepc). The two public keys may be the same or different (more secure).

As for steps a) through f), the functionality to perform steps g) and h)for the first software module of the set of software modules may beimplemented in hardware. The (piece(s) of the) hardware that providesfunctionality to perform steps g) to h) may therefore be equallytamper-protected.

Furthermore, the functionality to perform steps a) through f)(respectively steps a) through h)) for a software module other than thefirst software module could be implemented in hardware or could beimplemented in software within the entity block of the software moduleexecuted prior to said given software module.

Furthermore, it should be apparent that the unsuccessful execution ofone or more of steps a) to f) (respectively a) to h)) results ininterrupting the bootstrapping of hardware. This interruption of thebootstrapping could result in the hardware becoming temporarily orpermanently unusable. Alternatively, depending on the functionality tobe provided by the firmware/software, the boot process could befinished, but certain functionality—in particular functionality whichhas to be immune against attacks and/or counterfeit—may be disabled bythe hardware.

In one exemplary embodiment of the invention, the bootstrapped hardwareis a tamper-protected hardware module, and the functionality provided bythe software modules (TRUSTLETs) includes at least one of:

-   -   an interface filter (API Filter) for cooperation with a host        device's filter controller and for controlling messages        exchanged via an I/O-interface of the tamper-protected hardware        module for interfacing with the host device comprising the        tamper-protected hardware module,    -   a license check module which implements “Identification Friend        or Foe” (IFF) to analyze the local tamper-protected hardware        module. This is helpful to indemnify the decision if the        tamper-protected hardware module is a legalized or counterfeit        hardware and which application the hardware is licensed to run,    -   an Initial Program Loader module (IPL Boot). This TRUSTLET makes        it possible to coordinate the boot process and to allow options        within same. It may for example contain an extended        Power-On-System-Test (POST) and may manage (parts of) the update        process of the tamper-protected hardware module. The Initial        Program Loader may be optionally provided with a simple (user)        interface able to allow user-selectable decisions and features,    -   an embedded operation system (micro) kernel to handle the        various operations of the tamper-protected hardware module's        application module,    -   the application for the tamper-protected hardware module        specifying the tasks and features same is able to do,    -   an implementation of an off-line transaction protocol for a        transfer of electronic tokens between two hardware-protected        hardware modules (respectively two host devices comprising        same), and    -   a handler (i.e. a piece of software or dedicated hardware        managing the respective task(s)) for termination of the off-line        transaction protocol in case of an off-line transaction        protocol's interruption, including a generation of a proof of        loss of electronic token in response to the off-line transaction        protocol terminating in an unfair state.    -   A handler for checking and maintaining (e.g. by performing an        on-line refresh of the trusted time-source—if necessary) a        trusted time-source comprised in the host device,    -   a handler for initialization of the I/O-key-unit for real-time        I/O-encryption and decryption of information exchanged via an        I/O-interface of the host device (umbrella        encryption/decryption),    -   a handler for compensation for a loss of electronic tokens using        a proof of loss of electronic token provided in response to the        off-line transaction protocol terminating in an unfair state (as        described in the priority application PCT/EP2011/001219 of the        same applicant), and    -   a handler for checking a semiconductor module's cocoon PUF        fingerprint comprising the tamper-protected hardware module.

The tamper-protected hardware module may for example store the softwaremodules in an internal or external memory. Moreover, the hardware couldreceive an update of one or more of the software modules from a thirdparty.

Furthermore, in another embodiment of the invention a certificate (seee.g. Type-V certificate below) is appended to the software module (e.g.firmware or software) which is signed by a Root Certification Authorityusing a private signing key of a signing key-pair. The public key ofthis signing key-pair is provided immune against counterfeit in thebootstrapped hardware, respectively the tamper-protected hardwaremodule. Please note that the certificate may already be appended to thesoftware modules upon generation.

It should be further noted, according to this aspect of the invention,that the invention is not only related to the generation of the softwaremodules and bootstrapping mechanism using same, but also provides thehardware and/or software to perform the generation of the softwaremodules, respectively a hardware to perform the bootstrapping mechanismusing same. Hence, further embodiments of the invention provide adevice, adapted to perform the generation of the software modules, andfurther a tamper-protected die or multi-die, a tamper-protected chip,and/or a device adapted to perform bootstrapping mechanism using same.

Design of the Tamper-Resistant Semiconductor Module (CASTOR)

As already indicated above, different aspects of the invention relate toimproving the tamper resistance of semiconductor modules respectivelythe hardware module (e.g. die/multi-die/chip packaged therein). Asemiconductor module may be for example a die/multi-die bonded onto acircuit carrier and packaged into housing. In general, tamper resistancecan be best achieved by building a programmable full custom ASIC. SuchASIC may be for example realized as a System-on-Chip (SoC) withintegrated processing capabilities and a non-volatile memory to storeelectronic secrets, tokens, keys, passwords, and digital certificates.

A tamper-protected semiconductor module may make use of a variety ofknown and/or new techniques to become tamper-resistant and makingreverse-engineering as well as micro probing a challenging and difficulttask. The property of tamper-resistance may not be treated as anabsolute security measure as it is known, every tamper-resistantsemiconductor module using today's known techniques for making ittamper-resistant can be completely reverse engineered (see S.Skorobogatov, “Hardware security: trends and pitfalls of the pastdecade”, University of Cambridge, 2009). An attack can be consideredsuccessful if all protected information (root-secret) as electronictokens, encryption keys, and software code is revealed. However, thedesign of the transaction system with its PKI infrastructure, the designof the off-line transaction protocol discussed above, and furtherhardware-based and software-based techniques to “harden” against attacksmay—in combination—prevent an attacker from gaining the equivalent ormore of the invested money used to break the semiconductor module. Thisis very likely to deter the attacker from attempts to attack the system.

Although model and design of the CASTOR discussed below in furtherdetail combines a variety of techniques to prevent tampering, it is tobe noted that not all techniques may need to be used simultaneously inorder to provide a tamper-resistant semiconductor module. Everytechnique may increase the tamper-resistance and sometimes may interactwith another technique to improve tamper resistance of the semiconductormodule. Further, especially the novel techniques to improve tamperresistance discussed herein represent individual aspects of theinvention, but also new uses of known techniques, such as a PUF, areproposed in some aspects of the invention.

Moreover, the techniques to improve tamper resistance of thesemiconductor module (CASTOR) are not limited to semiconductor modulesfor use in a transaction system, but may readily be used in order toimprove the tamper resistance of a semiconductor module intended for anyapplication, e.g. in computers, cars, Smartphones, PDAs, PADs,smartcards, gaming consoles, PoS terminals, etc. just to name a few.

Model of a CASTOR Chip

FIG. 1 shows an exemplary block diagram of a CASTOR chip according toone exemplary embodiment of the invention. The IC/die is referred to asthe CASTOR chip while the packaged CASTOR chip is referred to as theCASTOR. However, in most cases this distinction inspired by theproduction process is not important for the overall understanding. Forsake of simplicity it is more often referred as CASTOR or a(tamper-protected) semiconductor module in most sections herein.

Exemplarily, the CASTOR chip comprises a main processor (CPU), e.g. anARM processor with an integrated interrupt controller (INT), a MemoryManaging Unit (MMU), a Direct Memory Access (DMA) unit, and a L1/L2cache. The chip may for example include a separate hardware interruptservice unit (INT/ISR). Additionally, a timer-unit and/or Clock PhaseLock Loop (Clock PLL), which may be used to implement timers fordetecting timeouts (e.g. fail-stop timeouts, lock timeouts, etc.), andto provide a WatchDOG timer (WDOG) in combination with a reset logic maybe provided. The CASTOR may further include power management circuitry(e.g. a sleep mode & wake-up logic, voltage & power regulator sensors,filters, etc.). The CASTOR also includes a volatile and non-volatileRandom Access Memory (RAM), e.g. an SRAM, DRAM, EEPROM, FLASH, MRAM,PCRAM, PRAM or the like. Optionally, the CASTOR may also be providedwith circuitry to generate and maintain (en-/decrypting, signing andchecking by recalculation) crypto objects (e.g. keys, unique objects andother security relevant objects)—see “Crypto Primitives” unit. Further,the CASTOR may also include a Scatter/Gather DMA speedup engine toimprove the DMA performance.

The CASTOR may further comprise an I/O-unit which allows to exchangeinformation (commands/messages and data) with its environment. Forexample, the CASTOR may be provided in a device and could use thedevice's memory or BUS functionality to communicate with another CASTORor another device/server/authority (e.g. the issuing authority (eMint),RCA, etc. as described in the priority application PCT/EP2011/001219 ofthe same applicant) through the I/O-interface. Alternatively, the CASTORmay also use tethering mechanisms for communication.

The circuitry providing the crypto-objects in combination with thecrypto-coprocessor (see below) and the scatter/gather DMA can form anefficient bulk cryptographic offload engine, able to collect packets andtheir fragments from multiple locations in system memory, and write backthe data upon completion of the cryptographic operation. This caneliminate demand for the main processor to require direct control on thecipher- and DMA-operations.

In addition to these elements of the CASTOR, the CASTOR may furthercomprise a crypto-coprocessor (Soft Core Maze of Knossos Processorimplemented as FPGA) which assists the main processor taking overcryptographic operations like primitive en-/deciphering, signing,verifying certificates and signatures, etc. Moreover, a PUF unit isproviding a hardware functionality, which may be used to verify theCASTOR chip's integrity. The off-line transfer protocol unit (asdescribed in the priority application PCT/EP2011/001219 of the sameapplicant) is a dedicated circuitry performing the off-line transactionprotocol, handling interruptions due to transmission errors and othertasks related to the transaction processing like the execution of anon-line fairness reconstruction process, obtaining system updateinformation from a trusted party, etc.

The optional key generation unit (PK/ID Generator & License Info) is acircuitry to generate keys, e.g. the session keys for a transaction,keys for the umbrella encryption, the CASTOR's public key-pair (SUDI andPUDI, the latter is contained in the CASTOR root certificate, asdescribed in the priority application PCT/EP2011/001219 of the sameapplicant), and numerous other keys required for the execution oftransactions and management functions in the system. Further, this unitmay also store the various generated keys. The key generation unit(PK/ID Generator & License Info) is an optional unit of the CASTOR, asfor example its functions could be also performed by the main processor(optionally in combination with the crypto-coprocessor and thecrypto-primitives unit).

Moreover, the CASTOR may comprise a bootstrapping unit (Init,Build-In-System-Test (BIST), TRUSTLET startup) dedicated to boot theCASTOR upon power-on or reset. The bootstrapping unit may for exampleperform a BIST and may control the loading of the firmware, e.g. usingsecure firmware modules (TRUSTLETs) as discussed herein.

The Process-Service-Security-Balanced-Interface (PSSBI) filtercontroller, which is another optional component of the CASTOR, isresponsible to control/filter the I/O-traffic of commands and dataaccording to the defined communication protocol with the external hostdevice.

Furthermore, the CASTOR chip may also be provided with a cocoon PUF,when packaging same.

Please note, not all components of the CASTOR shown in FIG. 1 are neededto provide a semiconductor module. Some functionality provided bydedicated circuitry (e.g. the off-line transfer protocol unit asdescribed in the priority application PCT/EP2011/001219 of the sameapplicant, the key generator unit, the crypto-coprocessor, thecrypto-primitives unit, etc.) may also be performed by the mainprocessor however, with less performance as if using dedicated circuits,optimized to the respective function or task.

Concept of the CASTOR Design Philosophy

The exemplary CASTOR shown in FIG. 1 is constructed to provide aneffective protection system, able to generate a kind of hard-to-clonechips in spite of mass production. To improve the tamper resistance ofthe CASTOR chip, the CASTOR chip may

-   -   contain a standard hardware (silicon, optical, magnetic,        acoustic, coating, butterfly, or intrinsic) hardware PUF,    -   contain a cocoon PUF,    -   be balanced by the Soft Core Maze of Knossos Processor,    -   contain a security monitor to enable the detection of potential        attacks,    -   generate its own identifying descriptor item (e.g. serial        number) crypto identity,    -   generate its own public key-pair (PUDI, SUDI as described in the        priority application PCT/EP2011/001219) to be used for digital        signing purposes,    -   generate an I/O-key (as exemplary private key component of a        PKI, e.g. the K_(E:CASTOR:I/O,sym) as described in the priority        application PCT/EP2011/001219 in the chapter “Private Key Rings”        of the same applicant) for the umbrella-encryption, and/or    -   be validated by certifying the identifying descriptor item (e.g.        serial number) by the RCA.

Using SoC processes to design and implement the CASTOR chip may be onetechnological means to create very secure systems. One reason is theachievable grade of integration: The smaller the chip structures theharder their reverse engineering becomes. Attackers routinely are usingsecond-hand semiconductor manufacturing and testing equipment onlycountered by the use of the latest highest-density technologies. Theapplication of unique-per-chip embedded secret storage in non-volatilememory in combination with a cocoon PUF and/or hardware PUF is among thebest of these techniques. Such combination is an advantage with respectto cost and effort for a physical attack because it raises the hurdle anattacker has to leapfrog with technical expertise, and financialresources sufficient to mount an attack.

Detected attacks against the CASTOR may be responded by temporarily orpermanently making the CASTOR unusable. This could be e.g. achieved byan instant erasure or—even better—by falsification of the CASTOR's rootcertificate or other essential CASTOR-internal objects, which are keyelements for the CASTOR to be operable in the transaction system. Inaddition thereto, essential functional circuitry and stored informationmay be partially and/or functionally self-destroying in a manner thatleaves the CASTOR in a state where it still seems to work, however in anerratic and wrong way. The destruction process can be realized bydriving high currents through internal circuit's logic conducts. Asilent self-destruction may be even more advantageous than indicatingthe perception of the attack as feedback to the attacker.

Expected Attacks on the CASTOR and their Countermeasures

This section discusses several possible attacks and a selection ofcountermeasures that may be implemented in the CASTOR or CASTOR chip.

Simple Power Analysis (SPA) and Differential Power Analysis (DPA)

With respect to SPA and DPA, one effective countermeasure may be the useof internal randomized power protection (adding random noise viainjection to internal power supply of the SoC) and jittering clocksignals in combination with (only partially) asynchronous self-timeddual-rail logic with alarm functionality by the CASTOR chip. It countersSPA and DPA by blurring and de-correlating statistical predictions. Eachbit will be signaled on two wires: (0,1) states “zero”, (1,0) signals“one” while everything else raises an alarm. There is a high probabilitya single transistor failure might also result in alarm propagation. Thepower supply may be internally filtered for fluctuation integration,realized through an ‘on-chip’ capacitor, which partially buffers theenergy consumption and reduces signal sizes. The additional usage of anidiot (idle) task (e.g. prime number generation) can be helpful, too.

Date and Time Manipulation

Another possible attack on the CASTOR may be based upon date and timemanipulation. The usage of certificates and CRLs, the determination ofthe electronic token transferability, and the prevention of all kinds ofreplay-attacks make the transaction system potentially prone to date andtime manipulation since all these objects rely on the system time,devaluating their meaning over time. One countermeasure may be theprovision of a trusted time-source on the CASTOR chip.

Glitches and Other Fault Injections

Further, the use of a self-clocking gate-design (which can prevent andlimit the delay and skew of the clock by an internal Phase-Locked-Loop(PLL), robust low-frequency sensors, a RESET filter, and over/-undervoltage protection are effective measures to counter glitch attacksintroduced through clock frequency manipulation.

Low-frequency sensors will trigger if no clock edge has been seen for aperiod longer than a specified time-limit. In this case, it may beadvantageous if parts of the CASTOR chip's internal logic are resetimmediately, and if bus lines as well as registers are erased/groundedquickly. An intrinsic self-test built into the detector prevents asimple disabling. An attempt to tamper with the sensor should lead to amalfunction of the entire CASTOR chip. Furthermore, external resets maynot directly be forwarded to the internal reset lines; it may be betterto cause the reduction of the clock signal by an additionalfrequency-divider which then triggers the sensor and leads to aninternal reset of the CASTOR chip, which finally deactivates thedivider.

Glitches, Electro-Static-Discharge (ESD) measures, and other fault-basedattacks are frequently injected with radiation (e.g. LASER UV light).Passive and active CASTOR shielding in combination with light detectorsand radiation-sensors will enable the design of a resistant chip.Situating the logic design inboard between metal layers makes this typeof attacks more improbable. Furthermore, an additional option is the useof a cocoon PUF.

Optical Scanning

On-chip busses and memory structures on the SoC are easily opticallyidentifiable. To prevent against an easy useful sniffing attack whichgathers information directly from the SoC-internal busses or cellmemories, busses and cell memories may be protected by encryption and/ordynamic address permutation. This may be a countermeasure which canprevent attacks on CASTOR chip internal data structures and transfers byECC, Error Detecting Code (EDC) and Cyclic Redundancy Check (CRC)protection mechanisms, or a sealing with signed values.

Electro-Magnetic-Analysis (EMA) and EMission SECurity (EMSEC)

One common attack against a chip is to senseElectro-Magnetic-Interference (EMI) emitted from the chip as “black-boxreconstruction-approach”. The CASTOR chip or CASTOR may therefore bedesigned in a way that reduces electromagnetic emanation by shielding(e.g. through a TEMPEST design) and/or obscured by faking the radiation.The protection provided by a shielding could be further enhanced by“salting” the SoC with additional oscillators. In one embodiment, theshielding is provided by a cocoon upon packaging, the cocoon providing acocoon PUF.

Laser Cutting

A further security measure may be the bonding of the CASTOR chip to thecircuit carrier and its encapsulation into a non-standard BGA packagewith custom markings. This allows preventing direct access to theexternal chip interconnections. Additional metal passivationsandwich-layers on the CASTOR chip may be a good measure to avoidcritical structures being exposed to the attacker without removingactive layers first.

In one exemplary implementation of the CASTOR chip, each layer isplanarized using a Chemical-Mechanical-Planarization (CMP) processbefore applying the next layer. As a result, the top metal layer doesnot show the impact of the layers beneath. The only way to reveal theirstructure is to remove (the) top metal layer(s).

A further security measure may be to render the chip's/die's layoutdesign virtually random, for example by conducting a simulatedannealing. Distracting components over multiple layers and locations incombination with the usage of own fixed (macro) structures not found ina public ASIC library will make the process of logical reverseengineering and reconstruction much more time consuming.

Moreover, the usage of meshed glue logic could further be used to makeit hard to find signals for physical probing. Modular redundancy ofcritical design parts makes it even harder for the attacker to launchsuccessful modifications through wire cutting and micro wiring(patches).

Further Design Optimizations for Improving Tamper Resistance

In order to provide a safe storage (e.g. for secrets/configurationdata), distributed One Time Programmable (OTP) memory in combinationwith embedded non-volatile (e.g. flash, EEPROM, MRAM, PCRAM, PRAM etc.)memories could be used on the CASTOR chip/die. They are physicallyintegrated on the same die all over the chip structure even lyingin-between different layers.

The structurally destruction (e.g. burning) of integrated debuggingports, unused access- and test circuitry (including interfaces) likeJTAG scan chains or BIST logic before the product release is anothermeasure which may improves tamper resistance of the resulting CASTOR.The destruction of such structures on the CASTOR chip/die preventsread-back or direct-write to internal memory cells through externalinterfaces. The destruction of these structures may be for examplerealized by burning them, e.g. by creating high currents from externalI/Os that will destroy the circuit structures.

Another optimization is to make memory states dependent on a number ofbits of physical storage, by distributing their logical meaning overmultiple different memory cells, so no single-transistor failure cansubvert the chip's security policy. It can be useful to seal the datausing CRC, ECC and EDC codes. As a result of data-remanence for memoryartifacts in some technologies (even by cooling down the die), allrelevant secrets and data stored on the chip may be continuously movedand inverted. Moreover, a low temperature sensor could be provided onthe chip for auto-powering down the CASTOR chip in case temperaturesbelow −40° Celsius (or similar low temperatures commonly not encounteredin nature or within the operating limits) are detected.

Further, it is possible to scan and snatch memory cell contents of allstorage technologies, including ROM, RAM, EEPROM, FLASH, MRAM, PCRAM,PRAM or the like by using Focused-Ion-Beams (FIBs), Laser-Cutters (LCs),Electron-Beam-Testers (EBTs), Electron-Microscopes (EMs),Atomic-Force-Microscopes (AFMs), Scanning-Probe-Microscopes (SPMs),Scanning Capacitance Microscopes (SCMs), Scanning Spreading ResistanceMicroscopes (SSRMs), Scanning Tunneling Microscopes (STMs), MagneticForce Microscopes (MFMs), Scanning Surface Potential Microscopes(SSPMs), Scanning Thermal Microscopes (STMs), Scanning ElectronMicroscopes (SEMs), and other tools.

To make such attacks as expensive as possible, one countermeasure may bethe usage of the smallest available SoC structures for the CASTOR chip.Furthermore, the use of on-chip-encryption and the realization of memorycells deep between the core layers make their direct read-out achallenging task for the hacker. These countermeasures may be furthersupported by precautionary flanking them with extra metal refractorypassivation sandwich on-die layers. Each of these layers may beplanarized using CMP before applying the next layer. Opaque top-metalgrids and shields as generic protective measure may further be used onthe chip to raise the bar for an attacker in terms of time and equipmentcost.

Another optional security measure is the usage of a die wrapping cocoon(different to the cocoon PUF). The naked CASTOR chip/die may beprotected by a thin high-resistance wire-based intrusion sensor toharden the chip de-capsulation process countering any further externalprobing. Such wrapping cocoon encloses the chip/die to be protected,while a very low current (in the nano Ampere range) is used to check thewrapping cocoon's physical integrity, if the protection wire remaineduntouched (opens and shorts). If the sensor line is broken, shortened toground, or electrical voltage or its resistance changes significantly,the chip/die may detect an attack and may take the foreseen procedureslike a temporary disablement of the chip/die or even a self-destructionof the chip/die. The wire of the wrapping cocoon may be folded back overitself and wound as multiple parallel strands to increase thesensitivity of the wrapping cocoon. A mixture of copper and silicon(potting material as similar as possible in both appearance andchemistry to the bond wire and the die) powder with ceramic cement maybe used to cement the wrapping cocoon in the package.

To prevent against attacker using IR LASER techniques, one possiblecountermeasure may be to replace bulk silicon with material nottransparent at useful LASER frequencies (e.g. Silicon/Metal Oxide(SiMOX), Silicon-on-Sapphire (SoS) or other Si-on-Insulator (Sol)technologies).

Trusted Boot Through TRUSTLETs

As already indicated above, one further aspect of the invention is theprovisioning of a secure bootstrapping procedure “for starting” a chipor device, also referred to as trusted startup. This bootstrapping isusing secured software modules (comprising executable code andoptionally data; also referred to as entity blocks below) provided bythe firmware. The provision of a trustable bootstrapping process may beof particular importance for chips (or devices containing same) thatshould be update-able. The trusted boot process discussed in thissection has a capability to device the chip's firmware into multiplesmall functional parts, each of them carrying out a specializedfunction. These secure self-contained fragments/software modules arecalled TRUSTLETs. It is apparent that a trusted boot by the use ofTRUSTLETs can thus improve tamper resistance of the semiconductor module(CASTOR).

The basic idea of the proposed secure bootstrapping process is simple:It identifies a systemic way of distinction in order to differentiatebetween a secure interior (a tamper-protected hardware module) and anunsafe exterior (the rest of the world). To achieve this, thebootstrapping model covers two different main organizational strategies:

-   1. The serial chain strategy, where one TRUSTLET passes the control    to the next TRUSTLET, where every TRUSTLET is started to be executed    only once every time, and-   2. the central chain strategy, where one (distinguished) TRUSTLET    (which may be the first TRUSTLET started in the bootstrapping    process) coordinates the execution of other TRUSTLETs. The other    TRUSTLETs are started to be executed only once every time and return    the control back to the (distinguished) TRUSTLET after their    execution.

It is also possible to have hybrid variations of both of theseorganizational principles. The expression “started to be executed onlyonce a time” intends to clarify that the exemplary TRUSTLET gets onlyone single start. It does not mean that (parts of) the code of therespective TRUSTLETs are not resident servicing the tamper-protectedhardware module during the work-phase.

The utilization of a trusted bootstrapping process using TRUSTLETs isnot limited to its use for chips to be employed as part of devices in atransaction system. The trusted bootstrapping by means of TRUSTLETs mayreadily be used in chips, semiconductor modules, and devices intendedfor any application, e.g. in smart-phones, computers, smartcards, gamingconsoles, just to name a few.

The trusted boot joins into the various security concepts presentedherein that can be used to make a chip or a semiconductor module inwhich it is packaged more tamper resistant. Furthermore, since onepossible scenario where the trusted boot can be employed are chips orsemiconductor modules for use in a transaction system (eCash system) aspresented in the priority application PCT/EP2011/001219 of the sameapplicant, and may form part of the overall security architecturetherein, it will be also referred to the integration of the trustedbootstrapping in the context of a transaction system. More specifically,the trusted boot by means of TRUSTLETs can be a part of a layeredsecurity strategy, which allows the establishment of trust between theCASTORs and the eCash system (e.g. represented by services fromRCA/CAs/eMints).

As will be outlined below in further detail, TRUSTLETs are compressedand encrypted entity blocks which allow checks of their integrity and ofthe authentic creation source. If a chip can be booted into anenvironment which honors security and compliant functions to accessrestricted data, the data integrity within the chip can be assured. Iftrusted bootstrapping cannot be assured, the chip or device comprisingsame is prone to boot it into an unsecure operating system, whateversecurity management system the secure kernel provides. Trustedbootstrapping ensures boot integrity protection because:

-   -   It can protect the chip against off-line attack vectors by an        authentication process,    -   It locks the chip, because if any entity block is tampered with,        the system will not boot,    -   It ensures that only an un-tampered chip is operational.

TRUSTLETs also facilitate secure firmware updates (even Over-the-Air).Further, TRUSTLETs are countering the following integrity attacks:

-   -   bootstrapping manipulation,    -   code manipulations of the operating system,    -   unauthorized generic code and data changes,    -   use of own (unauthenticated) code.        TRUSTLET Generation Process

FIG. 2 shows an exemplary process for the generation of TRUSTLETs by theRCA according to an exemplary embodiment of the invention. The softwarepieces (entity blocks) of the firmware may be for example generatedusing the standard tool-chain of compiling and linking. An entity blockis a piece of executable code (e.g. comprised in a code section of theentity block) and optional data (e.g. comprised in a data section of thecode block). In one exemplary embodiment of the invention, the entityblock is the resulting data of the linking process performed by alinker, but the entity blocks are not limited thereto. The data that maybe optionally comprised in the entity block are for example data staticdata (e.g. constant variables for the runtime system), and/or data forinitialization of parameters/variables (e.g. predetermined variables ofthe runtime system), etc.

In a first step, an entity block to be “transformed” into a TRUSTLETwill be hashed using a hash function (e.g. SHA1). The entity block maybe optionally compressed—it should be apparent that the hash-fingerprintis determined from the uncompressed entity block.

The hash-fingerprint of the entity block is then signed by encrypting itwith a RCA's private key for encryption (as exemplary private keycomponent of a PKI, e.g. the KE:FW:T1-XXX.prv as described in thepriority application PCT/EP2011/001219 in the chapter “Private KeyRings” of the same applicant) to generate a digital signature thereof.For example, an Elliptic-Curve-Digital-Signature-Algorithm (ECDSA) couldbe used for encryption, but also other asymmetric encryption algorithmsmay be used. This digital signature is then combined with the(compressed) entity block.

Hereafter, this linked package (containing the (compressed) entity blockand the digital signature) is again encrypted with a random key, e.g. asymmetric key. For this, AES (e.g. with a 256 bit key) could be used inthe XTS encryption-block cipher-mode. However, other symmetricencryption schemes and/or cipher-modes could be used, too. The randomkey is then encrypted with a RCA's private key for encryption (asexemplary private key component of a PKI, e.g. the KE:FW:T2-XXX.prv asdescribed in the priority application PCT/EP2011/001219 in the chapter“Private Key Rings” of the same applicant). Please note, the RCA may usethe same or different private keys for encrypting the hash-fingerprint(signing) and encrypting the random key (encryption). The encryption ofthe random key may use for example ECC (e.g. with a 571 bit key) oranother asymmetric encryption scheme.

Please note that parts of the following text and in the whole documentuses concepts (like public “KD” keys and private “KE” keys, other keyslike SUDI, PUDI, CON, PIO, the RCA, Type-V certificates, RCLs and thelike), an exemplary implementation of which are described in thepriority application PCT/EP2011/001219 of the same applicant (cf. interalia chapter “Public Key Rings” there).

The private key or keys used by the RCA to encrypt the hash-fingerprint(signing) and the random key (encryption) is/are owned by the RCA, whileits/their public key counterpart(s) is/are contained in the CASTORproduct (firmware) certificate (Type-V). In the final building step, theconcealed entity block is combined with the encrypted random key to formthe TRUSTLET.

Optionally, the RCA may include to each TRUSTLET (or only one for allTRUSTLETs constituting the firmware), the CASTOR product certificate(Type-V) which contains the public key(s) required for the decryptionand validation of the TRUSTLET at the respective chip (or device)executing the TRUSTLET. Alternatively, the CASTOR product certificate(Type-V) may also be made available to the chips (respectively, devices)by some other means.

Use of TRUSTLETs in the Trusted Bootstrapping Process

Before being executed or read, each TRUSTLET is validated using theprocess described below with respect to FIG. 3. FIG. 3 shows anexemplary embodiment of a validation process for a TRUSTLET which hasbeen produced according to the procedure described above with respect toFIG. 2. As discussed above, the TRUSTLET comprises two main parts: Theencrypted random key and the concealed entity block. To validate theTRUSTLET, the bootstrapping chip/device is first using the counterpartpublic key (KD:FW:T2-XXX.pub) to the RCA's private key for encryption(KE:FW:T2-XXX.prv) to decrypt the random key (symmetric key) which hasbeen used previously to encrypt the (compressed) entity block and itsdigital signature. If successful, the random key is subsequently used todecrypt the concealed entity block comprising the (compressed) entityblock and its digital signature. Next, the bootstrapping chip/device isverifying the digital signature of the (compressed) entity block. Forthis purpose, the entity block is decompressed (if it is included incompressed format to the TRUSTLET) and a hash-fingerprint is calculatedfrom the (decompressed) entity block using the same hash function asused by the RCA when creating the entity block's fingerprint. Further,the digital signature is decoded using the counterpart public key(KD:FW:T1-XXX.pub) to the RCA's private key for encryption(KE:FW:T1-XXX.prv) to obtain a reconstructed fingerprint of the entityblock. The reconstructed fingerprint and the freshly generatedfingerprint are compared and if they match, the entity block has beensuccessfully validated and is allowed being executed by the chip.

It should be apparent that any attempt to modify the contents of theTRUSTLET would falsify the validation process and interrupt thebootstrapping process.

It is worth noting that the functionality to validate the first TRUSTLETto be executed in the trusted bootstrapping process is hardcoded in thechip.

Moreover it should be noted that the counterpart public key(KD:FW:T2-XXX.pub) and the counterpart public key (KD:FW:T1-XXX.pub) maybe identical, although it may be more advantageous to use distinct keysfor signing and encryption by the RCA. The counterpart public key(s)used in the validation process may be for example provided to the chipfor example by means of a certificate, such as for example the CASTORproduct (firmware) certificate (Type-V) previously herein. Thiscertificate may be for example appended to the TRUSTLETs (or may be partthereof) or may be provided to the chip by some other means, e.g. bydownloading it on-line from the RCA.

Execution of a TRUSTLET starts through a previous TRUSTLET in the(logical) chain or, if the TRUSTLET is the first TRUSTLET to beexecuted, with functionality hardcoded in the chip itself as alreadystated above. Execution means the following herein:

-   -   a measurement of the current code/data state: checking the        authenticity,    -   case yes: CASTOR passes the execution control to the measured,        decrypted and decompressed TRUSTLET,    -   case no: CASTOR performs a RESET. In such case the CASTOR starts        over with the same procedure as performed by a power-on-event.        Alternatively to that method the following techniques may be        employed: Entering into an error mode (and the indication of        same to the user) or performing a (deep) sleep or power-downing        the CASTOR.

This execution is performed with every TRUSTLET within the chain. Pleasenote, there are many possible different logical chain configurations.Three possibilities are depicted in FIG. 15, FIG. 16 and FIG. 17. Thechain position of every TRUSTLET is hardcoded during thecompilation/generation phase of the chip's firmware, so every TRUSTLETknows it's succeeding TRUSTLET. During its boot phase, the CASTOR cyclesthrough the chain of all of its firmware's TRUSTLETs. The chain of trustis kept alive as long as each TRUSTLET ensures any subsequent TRUSTLETis “executed in the way as explained above”.

This structured execution assures neither off-line system tampering noran attempt to boot an unauthorized firmware can happen. A trustedstartup employing TRUSTLETs protects the system boot process by ensuringtamper-freeness before releasing the system control to the embeddedoperating system and granting system access to the protectedapplication. Trusted startup enabled system secrets could be for exampleprotected by using the chip to encrypt root-secrets, rather thanprotecting through obfuscation or by shared passwords.

The trusted startup protected chip is resilient against advanced methodsof off-line code and data modification on the secured storage. Assumingthat the chip is tamper-protected (using one of the various measuresdescribed herein), i.e. provides the hardware protection for the startupintegrity, decryption keys, and the sealed storage, every attempt tochange a boot sequence component makes the firmware inaccessible, thesystem unbootable, and the application data unattainable (as beingprotected by the application of an umbrella encryption).

The functionality for the provisioning of the umbrella encryption may beimplemented e.g. as a TRUSTLET, too. Therefore, not only a chip-internalstorage may be encrypted, but also data stored outside the chip can beencrypted by means of the umbrella protection, as described for theCASTOR chip herein. Because memory content and external stored dataobjects may be encrypted in real time while being transferred to theexternal mass storage device through secured-I/O, data objects likeeCoins, certificates, CRLs, and any (open) document or (cached) secrets(and e.g. even swap files) are also encrypted under this umbrellaprotection. Therefore, the mass storage data can be encrypted completelyand is therefore not exploitable. Off-line attacks cannot compromise orreveal system passwords, keys, or any other data. In addition, replay orimage duplication attacks (launched by the attacker by copying thecontent of the external memory with the encrypted eCoins and intendstheir reuse) are thwarted by synchronization checks to the last safesealed transaction state using the chip-internal memories. By comparingcertain boot process characteristics to previously stored measurements(e.g. signed cryptographic hashes), a trusted startup through TRUSTLETscan detect system tampering while starting up. This enables the trustedstartup to verify system integrity at an early stage, improves datasecurity, and reduces equipment-re-purposing concerns. The feature issimple to deploy and operate, and it enables an easy system management.

Although it may be advantageous to interrupt the trusted bootstrapping(leaving the chip without a usable embedded operating system) in case aTRUSTLET cannot be validated successfully, this interruption may notalways be feasible or desirable depending on the target application.TRUSTLETs may contain only individual functions of the firmware,security relevant ones as well as security irrelevant ones. Therefore,in another embodiment of the invention the chip may also be able tocontinue bootstrapping with the subsequent TRUSTLET in chain, even ifone or more TRUSTLETs cannot be validated, thereby possibly being ableto boot at least a partially operable system. Thus, the functionality ofthe non-executed TRUSTLET(s) (due to a validation error) won't beavailable to the system.

Furthermore, another option may be that an entity block of a TRUSTLETcomprises multiple code sections with executable code and optionallyfurther data sections. In case the TRUSTLET cannot be validatedsuccessfully, a specifically dedicated code of one code section could beexecuted, while in case the TRUSTLET is validated successfully, anothercode of another code section is executed. The code of the respectivecode sections (and the corresponding data in the data sections) may beadapted to the specific needs and application of the chip or system inwhich the trusted startup is used. For example, the code of the codesections executed in cases where the TRUSTLET cannot be validatedsuccessfully could ensure the bootstrapping of the chip with a minimaloperating system of the chip which is for example not allowing anytransactions requiring a secure environment. The code of the codesections executed in cases where the TRUSTLET is validated successfullycould be for example ensure the bootstrapping of the chip with thefull-functional operating system of the chip allowing to perform alsotransactions requiring a secure environment.

A further option to react to a validation error of a TRUSTLET may bethat the chip (e.g. its boot manager) continues booting the systemand—without allowing any security relevant operations—notifies the RCA,if it encountered a non-validated TRUSTLET on the chip/device. Thisnotice to the RCA may for example include an identifying descriptor item(e.g. serial number) of the chip, which would allow the RCA to lock thedevice by (black-) listing its identifying descriptor item (e.g. serialnumber) in a respective RCL, assuming the chip's identifying descriptoritem is provided in form of a certificate. Upon having notified the RCA,the chip may then disable itself or the entire device.

Optionally, instead of a sequential bootstrapping process where oneTRUSTLET validates and starts the next TRUSTLET to be executed, thebootstrapping process may be changed as follows: The first TRUSTLET tobe executed may provide a boot-manager's functionality (please refer toFIG. 16) which controls the execution of the subsequent bootstrappingprocess. Please note, the boot manager may be part of the hardware ormay be implemented as part of the hardware functionality. The bootmanager may thus control the execution of (further) TRUSTLETs. For thispurpose the boot manager provides the necessary functions to validateTRUSTLETs as described above with respect to FIG. 3, and some decisionlogic to decide how to proceed (in) the bootstrapping process (or how tointerrupt same) in case a validity check fails for a TRUSTLET. Further,the boot manager may of course be further aware in which order theTRUSTLETs should be executed.

Trusted Bootstrapping of a Chip/Device for Use in the Transaction System

FIG. 4 shows exemplified a bootstrapping process according to anexemplary embodiment of the invention, which specifically relates to theuse of the trusted bootstrapping for a device (respectively, the CASTORchip) used within a transaction system as described herein:

-   1. After power-on, the CASTOR hardware (i.e. the chip to be    bootstrapped) is initialized, which may for example include a basic    BIST internal hardware functionality test. This initial BIST may be    later on augmented with a more comprehensive test. Thereafter, a    first TRUSTLET (being the API filter in this example) is executed as    exemplified above. All these actions are performed by the hardware;    no software is involved at this stage.-   2. Given the first TRUSTLET (in this specific case the API filter)    is successfully executed; the subsequent TRUSTLET will be executed    next. Each TRUSTLET may comprise code (see FIG. 15 and FIG. 17) to    perform the execution process described with respect to FIG. 3 for    the next TRUSTLET or only a “managing TRUSTLET” may comprise that    code (see FIG. 16). Accordingly, at the “end” of the execution of    the first TRUSTLET, this first TRUSTLET will execute the next    TRUSTLET only in case the next TRUSTLET can be successfully    validated. This procedure is continued until the last firmware's    TRUSTLET is executed. As each TRUSTLET checks the validity of the    next TRUSTLET prior to its execution, a chain of trust is generated,    and it is not possible to sneak unauthorized code into the    bootstrapping process (of course presuming that the PKI    infrastructure is not broken, which is extremely unlikely if not    impossible). In case a TRUSTLET is not validated successfully the    bootstrapping process may be interrupted. In case the API filter    executes successfully, in turn, the License Check TRUSTLET is next.-   3. The License Check TRUSTLET is responsible for performing a more    comprehensive system test, e.g. by checking the chip's (or the    semiconductor module's) integrity using one or more of the further    security means to detect tampering of the chip or the semiconductor    comprising same, such as e.g. checking the integrity by using the    built in hardware PUF, the cocoon PUF and/or a configuration check    of the crypto-coprocessor, etc., respectively. Furthermore, the    License Check TRUSTLET authenticates the chip's license information    as provided in the respective section (e.g. Cert.Unit.Info.CLC) of    the CASTORs Type-VI RCA root certificate (see section “CASTOR Root    Certificate (Type-VI)” of the priority application    PCT/EP2011/001219).-   4. One possible (but not exclusive) way to achieve this is    cross-referencing the information provided by the RCA root    certificate with other—later on delivered—on-line or off-line    available information provided by the RCA to the CASTOR (e.g. if    licensee do not pay its license fees or is not a certified customer    of the system anymore). In case the License Check executes    successfully, in turn, the IPL (Initial Program Loader) Boot    TRUSTLET is executed next.-   5. The IPL Boot TRUSTLET is the bootstrap service loader for the    (embedded) operating system. It performs the extended    Power-On-System-Test (POST) and manages the update process of the    chip's firmware. In case the IPL Boot executes successfully, in    turn, the (embedded) operating system TRUSTLET is next.-   6. The (embedded) operating system TRUSTLET is the operating micro    kernel responsible for the on-CASTOR management. After its    initialization this software is responsible to load the application.    In case the (embedded) operating system executes successfully, in    turn, the application TRUSTLET is next.-   7. The last TRUSTLET in the chain will be the application. In one    embodiment of the invention this is the eCash transaction system    application.

It should be noticed, any executable code not validated (e.g.non-validated TRUSTLETs or standard code) is impossible to be executedin such environment (excluding the above mentioned “relaxed” securityhandling possibilities). A chain of trust can only be assured by theexclusive use of TRUSTLETs. If ever the execution chain “forks” to aninvalid TRUSTLET the chain of trust ends with the last validatedTRUSTLET. Henceforward, control of the CASTOR within a guaranteed secureenvironment is not possible. As already mentioned earlier in thissection this can make sense due to the nature of the (intended)application e.g. with no or only negligible needs for security.

The “logical chain of trust” which provides trusted boot capabilityimplemented by the TRUSTLET system can be technically encompassed invarious architectural ways. FIG. 15 depicts the simplest linear model.As shown in FIG. 16, also a central approach is possible, where a“managing TRUSTLET” (e.g. the IPL boot TRUSTLET) sits in the center ofthe boot process. Countless hybrid forms are also possible; one of themis exemplary depicted in FIG. 17.

Exemplary TRUSTLET: API-Filter

In the transaction system described above the CASTOR chip communicateswith an external host, i.e. the device containing and/or operating theCASTOR. The CASTOR's host may never even provide an operating platformthrough a (graphical) user interface. However, its communicationcapabilities can nevertheless be used to communicate with the external(remote) world.

The interface between the CASTOR chip's I/O-unit and the host may followa point-to-point architecture consisting of two participants, thesecurity service provider (CASTOR chip), and a generic service consumerdevice (e.g. PC, smart-phone, TV, Car, eFilling station, paddle,separate eWallet etc.). The interface of such architecture is sometimesalso referred to as a Process-Service-Security-Balanced-Interface orPSSBI (general outlined in FIG. 6) and is provided by the API FilterTRUSTLET.

According to a certain policy, conventional security APIs provide acommand set using cryptography to control the processing of and theaccess to sensitive data. The tasks of a security API are to allow usersto process data and key material, achieve flexibility and atomicityaccording to the designer's intention, and prevent any malicious commandsequences which might violate these intentions. It provides acommunication interface for commands on a very low level (meaning toprovide only basic and simple operations) with the intension of beingusable in a more general way.

In contrast, according to an embodiment of the invention, the API filterprovides a communication filter interface for services on a very highlevel (e.g. application specific level). Data is exchanged over thisPSSBI interface in form of Protocol Data Units (PDUs) located between ahardware module and the external host (please take a look to FIG. 7).There are two types of PDUs:

-   -   command PDUs containing data sent to the CASTOR chip, and    -   response PDUs containing data sent by the CASTOR chip.

To prevent some sort of abuse attacks, the API Filter TRUSTLET furtherenhances the CASTOR chip (owning the PSSBI Filter Controller as hardwareresource) with a PSSBI gatekeeper, which

-   -   checks incoming PDUs for their correct formats,    -   formats outgoing PDUs, and    -   calls the respective internal CASTOR chip service based on the        PDU received from the host.

Optionally, the PSSBI gatekeeper may provide protection against bufferoverflow attacks and may further detect, repeal, and recover from known(interface related) attacks.

PUFs

The application of a Physical Unclonable Function (PUF) is a technologyin which an easy to evaluate function, embodied in a physical structure,maps a set of challenges to a set of responses based upon a hard tocharacterize complex physical system. The physical structure of a PUFconsists of many random components introduced during the manufacturingprocess in a not controllable fashion. They can be regarded as digitalfingerprint or “digital DNA” of a chip-identity. The PUF can help toenhance a system's security because parts of or even the complete rootsecret can be stored and protected without the application of digitallystored information.

As depicted in FIG. 5, a PUF can be regarded in functional comparison toother mapping techniques like the ROM or a crypto function. Thedisplayed ROM maps 2128 different ‘challenges’ to a 128 bit wide‘response’. If the ROM matrix is filled with white noised data an almostperfect digital mapping with two exceptions is achieved: It will neverbe possible to build a ROM able to carry more bits than the number ofstars in the universe, (2128)*128 bits as required in this case. Andeven in case this was possible it would be susceptible to a copy attack.

If a cryptographic function like 3DES, AES or others is used, suchfunction can replace the ROM mapping device. This is usually done inevery Electronic Codebook (ECB) encryption block cipher mode ofoperation, the simplest one. The problem with such approach for a PUF isthat there is no copy protection of the crypto cipher at all (once thecrypto function becomes known, the protection is gone).

PUFs inherit their un-clonability property from the fact every PUF has aunique and unpredictable way of mapping challenges to responses in aniterative way. Due to the manufacturing process, even dies located onthe same wafer (e.g. used for silicon and intrinsic PUFs) still exhibita unique challenge-response behavior. PUFs are useful in authentication:Due to the presence of these random components, the PUF reacts in anunpredictable way (the corresponding response) if a physical stimulus(the challenge) is applied. The un-clonability property makes isimpossible to build a PUF with identical challenge-response behavior asobserved from a given PUF. In PUFs, different sources of physicalrandomness can be used. A distinction is made between PUFs in whichphysical randomness is explicitly introduced and PUFs using randomnessintrinsically present in a physical system.

According to one aspect of the invention, a PUF implemented in is usedto validate the integrity of a hardware module (which may be a die,multi-die or IC). A hardware module can also be referred to as a chip.In some embodiments of the invention, the chip comprises a hardwarestructure providing a PUF. To use the PUF for integrity checking, it isprovided with a set of challenges and the PUF's responses thereto arestored as pairs of challenges and corresponding responses.

The integrity of the chip can be checked by applying a set of challengesto the hardware structure of the chip providing the PUF and by receivingthe PUF's responses to these challenges. For at least one challenge, thePUF response is known from the previously stored set of PUF challengesand corresponding PUF responses. A mismatch between the known PUFresponse for a challenge and the actual PUF response received from thehardware structure of the chip indicates a tampering or cloning of thetested chip.

Such PUF based integrity check may be used in different fashions.

In one exemplary implementation, the integrity check based on the PUFcould be chip internal. In this case the challenges and theircorresponding known PUF responses should be recorded and provided by atrusted party (e.g. RCA) in form of a certificate, which is stored innon-volatile memory inside the chip. By validating known challenges andcorresponding known PUF responses, the chip can assure the challengesand corresponding known PUF responses haven't been tampered.

In this exemplary implementation, the chip could provide some integritychecking function, which may be part of the firmware in charge to checkthe integrity of the chip in given regular or random intervals (and atthe least at bootstrapping). The integrity checking function may selectan arbitrary challenge from the validated pool of challenge/responsepairs and tests the PUF for the correct PUF response.

The PUF may also be used to allow for an authority to check theintegrity of a chip. In this case the authority (e.g. host device, theRCA, an institution manufacturing a product containing the chip with thePUF, etc.) has recorded a set of PUF challenges and their correspondingPUF responses. For integrity checking, the chip would be provided withone or more challenge to which the response is known to the authority.If the PUF response(s) provided by the chip is/are correct, theauthority can confirm the integrity of the chip, or the host device,respectively. Otherwise the authority could e.g. cause a deactivation ofthe chip respectively its host device, an action also feasible by meansof a certificate revocation through a CRL. In one advancedimplementation, the challenge(s) for which the PUF response is knownis/are sent to the chip as part of a set of challenges, so the chipcannot know which of the challenges is “hot”, i.e. for whichchallenge(s) the querying authority knows the appropriate PUFresponse(s).

In one exemplary embodiment of the invention, the PUF is implemented ina CASTOR chip for use in a transaction system as described herein. Inthis exemplary embodiment, the RCA may record the challenges and theircorresponding PUF responses as part of the CASTOR'scrypto-initialization at the RCA (for more details, see sectionCrypto-Initialization of the Tamper-Protected Hardware Module (CASTOR)).The PUF may be used by the RCA to verify integrity of the CASTOR or mayalso be used by the CASTOR chip internally for verification of theintegrity as described above.

Chip-Internal Hardware PUFs

The (standard) hardware structure provides a response to challengesinput into the hardware PUF. Furthermore, the tamper-protected hardwaremodule may be equipped with a processor unit for providing at least onechallenge to the hardware structure implementing the hardware PUF, who'scorrect (cryptographic hash of) response from hardware structureimplementing the hardware PUF is known. The processor unit receives arespective (cryptographic hash of) hardware PUF response for eachchallenge provided to the hardware structure implementing the hardwarePUF and verifies integrity of the tamper-protected hardware module bychecking whether the (cryptographic hash of) response to at least onechallenge (for which the correct (cryptographic hash of) hardware PUFresponse is known) is matching the (cryptographic hash of) responsereceived from the hardware structure implementing the hardware PUF.

In one exemplary embodiment of the invention the processor unit of thetamper-protected hardware module provides plurality of challenges to thehardware structure implementing the hardware PUF. The plurality ofchallenges including the before-mentioned or more challenges for whichthe correct (cryptographic hash of) hardware PUF response is known. Thismay be advantageous as the processor unit cannot recognize whichchallenge(s) response(s) to the hardware PUF (and their cryptographichash(es), respectively) is/are known already. Therefore, it is unable todifferentiate between a fake challenge and a critical one.

According to a more advanced embodiment of the invention thetamper-protected hardware module further comprises an I/O-interface forreceiving at least one challenge for which the correct (cryptographichash of) hardware PUF response by hardware structure implementing thehardware PUF is known. The respective (cryptographic hash of) thehardware PUF response for each challenge applied to the hardwarestructure implementing the hardware PUF could be then made availableoutside the tamper-protected hardware module through the I/O-interface.

This would e.g. allow external authorities or devices to check theintegrity of the tamper-protected hardware module. Hence, thechallenge(s) to the hardware PUF could be received from a devicecomprising the tamper-protected hardware module (i.e. from an “externalcomponent” of the device) or on-line from another, external device.Likewise, the respective (cryptographic hash of) hardware PUF responsesfor each challenge provided to the hardware structure implementing thePUF could be provided to the device comprising the tamper-protectedhardware module or on-line to the external device, respectively.

An integrity check of the tamper-protected hardware module utilizing thehardware structure providing the PUF may for example be atamper-protected hardware module function, implemented by the firmware(e.g. upon power-up or reset of the tamper-protected hardware module).For this purpose, in another embodiment, the tamper-protected hardwaremodule may further comprise a storage unit for storing a set ofchallenges and their (known) corresponding (cryptographic hash of)hardware PUF responses. In one exemplary embodiment, the challenges andtheir corresponding (cryptographic hash of) hardware PUF responses aresigned by a Root Certification Authority to allow verification of theirintegrity. The tamper-protected hardware module may verify the integrityof at least one challenge and its (cryptographic hash of) hardware PUFresponse based on its signature. The challenges and their corresponding(cryptographic hash of) hardware PUF responses could for example besigned by the Root Certification Authority using a signing key-pair'sprivate signing key, while the public key is provided immune againstcounterfeit in the tamper-protected hardware module, is used to verifythe integrity of the challenges and their corresponding (cryptographichash of) hardware PUF responses.

Moreover, another embodiment of the invention relates to atamper-protected semiconductor module provided with a tamper-protectedhardware module having a hardware structure implementing a PUF, asdiscussed above. Further, another embodiment of the invention provides adevice which comprises such tamper-protected semiconductor module.

The type of the hardware PUF can be of standard hardware (silicon,optical, magnetic, acoustic, coating, butterfly, or intrinsic) PUF, oreven a cocoon PUF.

Semiconductor Modules with Cocoon PUFs

Another aspect of the invention is related to the design of thetamper-protected semiconductor module. Please note, this aspect isindependent of the other aspects of the invention provided herein,but—in some embodiments of the invention—may nevertheless be used incombination with them to further improve tamper resistance.

The basic approach underlying this aspect of the invention is to designa semiconductor module which has unique properties with respect to thedeflection of a physical measurand within the semiconductor module inresponse to an excitation (which could be also denoted a challenge).These unique deflection properties inside the semiconductor module areintroduced by the production process of the semiconductor module (forexample as a result of its manufacturing process) and are thus capableof uniquely identifying the semiconductor module. Further, any attemptto tamper the semiconductor module will alter its deflection propertiessince any attack on the inside of the semiconductor module willinfluence the physical measurand. This makes tampering attemptsobservable by the semiconductor module. Therefore, it is referred to as“tamper-protected”.

FIG. 11 shows a tamper-protected semiconductor module according to anexemplary embodiment of the invention, which is provided with a cocoonPUF. The semiconductor module is made of a hardware module (die/chip).The hardware module of the semiconductor module is exemplified in FIG.12 and comprises one or more emitters and one or more sensors (alsoreferred to as an emitter-/receiver phalanx herein). The emitters emit apredetermined excitation which can be measured as a physical measurand.The one or more sensors of the hardware module sense the (back-) scatterof the predetermined excitation in form of the physical measurand in acontactless and/or contact-based manner. As shown in FIG. 11, thehardware module is bonded (e.g. by means of a ball grid) to a circuitcarrier providing terminals to the outside of the housing. Anotherexemplary tamper-protected semiconductor module is shown in FIG. 22,where a cross-sectional view of a tamper-protected semiconductor modulebased on a BGA packaging is shown. The bonding and the provision of theconnection terminals may be realized by any suitable technique knownstate of the art.

The hardware module protected by the cocoon may be any hardwarestructure that should be made tamper resistant. In one exemplaryembodiment of the invention, the hardware module is a die, a multi-die,or a chip comprising one or more Integrated Circuits (ICs). Likewise andin another embodiment of the invention, the hardware module is a die, amulti-die, or a chip comprising one or more ICs assembled on a circuitcarrier (e.g. by means of a bonding process).

The tamper-protected semiconductor module further has a cocoon thathouses the hardware module (housing). This cocoon may also be referredto as a package or housing. The cocoon is configured to influence the(back-) scatter of the predetermined excitation to be sensed by thesensors. In the example of FIG. 11 the cocoon comprises a shieldingwhich is advantageously provided towards the inside of the housing andwhich is surrounding the entire hardware module and the circuit carrier.The shielding is preventing interference with the measurand from theoutside of the semiconductor module. Furthermore, the inside of thehousing (and inside the shielding), the (gap/cavity inside the)semiconductor module (between the hardware module and the cocoon) may befilled with a potting material or—in another implementationpossibility—the housing is the cocoon and thus contains the pottingmaterial, leaving the gap/cavity free, e.g. filled with gas.

Further, the housing may be non-transparent in the optical and infraredspectrum. This feature may be for example advantageous for preventingnon-invasive attacks on the inside of the tamper-protected semiconductormodule (respectively, its cocoon as the outer part), e.g. by means ofinfrared light or other optical scanning methods. Moreover, the cocoonmay have a high thermal conductivity, so that heat (generated by theoperation of the hardware modules circuitry and/or circuit components)may be guided to the outside of the cocoon thus enabling cooling of thehardware modules from the outside of the surrounding cocoon. A highthermal conductivity may be defined as a thermal conductivity λ>10W/m·K.

In order to provide unique deflection properties inside the cocoon, thedesign of the cocoon (besides adding a shielding) may ensure for suchproperty. For example, the cocoon comprises a coating and/or a pottingcompound including a material or material mixture (e.g. granules addedto a conventional coating/potting material, such as a ceramic or asynthetic material) influencing the (back-) scatter of the predeterminedexcitation. Please note the coating and/or potting is provided insidethe shielding. In the exemplary embodiment shown in FIG. 11 the pottingmaterial is provided with some material (here, granules) whichinfluences the (back-) scatter of the predetermined excitation of theemitters. In this embodiment, an electromagnetic excitation is used,i.e. the sensors may for example sense the electromagnetic field (e.g.magnetic field intensity and/or electric field intensity) of the (back-)scatter of the predetermined excitation from a specific point or regionof the cocoon emitted from the emitters on the hardware module, whichcould be viewed as charting of an excitation map. These measured valuesare then quantized and form the digital PUF “response” to the“challenge”, i.e. the predetermined excitation.

The granules within the potting and/or coating are of a material whichreflects the excitation within the limits of the cocoon (i.e. theshielding) so that a unique (back-) scatter is sensed by the sensors.The distribution of the granules within the potting material isvirtually random and depending on the production process of the cocoon.Therefore, the (back-) scatter obtained in response to the predeterminedexcitation is unique to the semiconductor module.

In another exemplary embodiment of the invention, the material of theshielding and/or the material of the granules thereof are similar to thematerial mixture of the hardware module and/or its bonding with respectto their chemical character (and optionally the physical character) tomake full-invasive attacks on the tamper-protected semiconductor modulemore difficult: The package (cocoon) of a semiconductor module can beremoved using acids or chemical bases in order to access the insidehardware module. By choosing the chemical character of the cocoonmaterial and the hardware modules material (and/or its bonding) to havesimilar chemical resistance against acids/chemical bases, it isdifficult to control the edging to only remove the cocoon material,while not damaging the hardware module and/or its bonding. Similarity inthe physical character may be advantageous, since the physical characterinfluences the reaction time with the acid/base. Hence, it becomes evenmore difficult to control etching precisely enough it only removes thecocoon material without damaging the hardware module or its bonding.

Basic Cocoon Mechanism of Action

As described above, one basic aspect of a (cocoon) PUF is the creationof a fingerprint using the cocoon. The fundamental mechanism of actionis the statistic distribution of detectable descriptors for exampleembedded in potting materials or laminator foils surrounding theprotected hardware module as housing. Exemplary implementations are madeout of plastic, ceramics or glass components based materials, buildingthe backbone of the cocoon.

There are variances of possibilities about how the fingerprintscharacteristics can be implemented, part of these are presented in thefollowing.

One qualitative umbrella target of the cocoon functionality is thedistribution and dispersion of events evidencing the existences ofinjuries and damages (penetrations) of the cocoon making holes (ideallyat any position in the whole) structural cocoon matrix detectable. Thisintroduces a threshold of perception.

The cocoon PUF reaches that goal by a passive (without anyenergy-consuming countermeasure to the attack) irreversible changing ofthe fingerprint.

In some embodiments of the invention, the following two approaches toachieve this are combined:

-   1. Granules or particles of solid objects and/or bubbles filled with    liquids or gases of different size and shape are statistically    distributed in a potting matrix. This mixture is then treated in way    that it will change its state from a malleable (for example liquid)    mass to a fixed state. This locates all the component parts inside    the mixture, forming the cocoon. The excitation used to form the    physical measurand(s) uses nearly all of the cocoon material, so    that the transfer function of the PUF will change in case of    “visible” cocoon changes (e.g. holes above the threshold of    perception). Due to a weak (e.g. electromagnetic, optical or    acoustic) coupling of the obstacles in the potting matrix, most of    the measured effects are from the local material and the receptors    for the measured data near to the position of the change (e.g.    hole). More far away situated receptors are not expected to    contribute much of the mechanical distortion and field displacement    caused by the breach.-   2. The “visibility” of breaches is enhanced, if the effect has the    ability to “grow”. This happens for instance if the structural    changes caused by the breach (e.g. hole) influences further    additional areas of its neighborhood so that the effect is    replicated and insofar amplified. The result: modifying a small    region of the cocoon structure has a correspondingly great effect on    the extracted fingerprint.    Memory Mechanism of a Cocoon PUF

According to an exemplary embodiment of the invention, the cocoon PUFuses the challenge-response-pair (CRP) method, but only internal, forexample by using a Fuzzy Extractor as shown in FIG. 21. Due to theprinciple of fingerprint generation, the fingerprint (FP) can bemeasured only from the inside of the cocoon. The existing and measurable(part of the) entropy will limit the number of different fingerprints tobe expected. This equals to the cardinality as being the same number ofelements as the CRP set. It is reasonable to expect more than 1020variations, which is definitely enough for practical applications.

The fingerprint of the cocoon PUF is fixed like a ROM, i.e. after thecocoon production it is generated and will never change (only in case ofan attack, as already said). To extend this ROM behavior to be usablefor arbitrary secrets, a non-volatile memory capability may beimplemented in the design. The raw fingerprint will be transformed afterits measuring into a long-term stable digital dataset by using e.g.error correcting measures to stabilize the fingerprint against smallvariations introduced by the analog nature of the memory. In embodimentsof the invention, the fingerprint is not stored in a digital memory, butrecreated as it is needed (by a new measurement process). This can bedone at every time, but only with the original, unchanged cocoon.

The arbitrary secret (SE) is encoded in the following way: The “owner”of the cocoon PUF calculates the function using the secret and thefingerprint (FP) as operands. For example the function can be realizedby a simple XOR operation (SE XOR FP), but also other optionally morecomplicated functions could be used. The result of this function iscalled Helper-Data (HD). The Helper-Data may then be stored in a(digital) read/write memory without the need for further protection. TheHelper-Data can thus for example be stored in- or outside of the moduleprotected by the cocoon PUF. This procedure happens at minimum once atime in the beginning of the application of the cocoon PUF, where forexample the IP owner “injects” the secret from the controlledenvironment outside. The reconstruction of the secret in the abovedescribed exemplary XOR operation, the calculation of (HD XOR FP)recreates the secret SE. Such a PUF usage model is also calledPhysically Obscured Key (POK) memory.

Different Variations of Principles of a Cocoon PUF

Since the properties of deflection, reflection, absorption, (back-)scattering, and refraction of the inside of the cocoon in combinationwith a given excitation depends on the production process, e.g. thepackaging process of the hardware module, the measured (back-) scatterof the predetermined excitation influenced by the cocoon (deflectionproperties depending on the production process of the cocoon) provides aphysical unclonable function (PUF) specific to the tamper-protectedhardware module. This PUF functionality of the tamper-protectedsemiconductor module is for simplicity also referred to as “cocoon PUF”.Using this cocoon PUF the hardware module is able to detect tampering ofthe tamper-protected semiconductor module and to detect non-invasive (orpassive), semi-invasive and full-invasive attacks to access the insideof the cocoon or the hardware module, respectively. In case the hardwaremodule detects a tampering attempt to the tamper-protected semiconductormodule, the hardware module could for example make itself temporarily orpermanently unusable.

How the tamper-protected semiconductor module is using its PUFfunctionality to detect tampering may be implemented in various ways.For example, in one exemplary embodiment of the invention, thetamper-protected semiconductor module may generate a digital “measuredfingerprint” susceptible to be represented as e.g. a map (i.e.containing positional information, denoting the fingerprint that has—soto say—just been “measured” by the sensors) of the tamper-protectedsemiconductor module from the measured values of the (back-) scatter ofthe excitation provided by the respective sensors and detects an attemptto tamper the tamper-protected semiconductor module by comparing themeasured digital fingerprint with a digitized fingerprint of theun-tampered tamper-protected semiconductor module (which may have beenfor example stored previously). It should be apparent that the repeatedverification of the integrity of the cocoon or the tamper-protectedsemiconductor module requires the same predetermined (map of) excitationis emitted in each verification process, i.e. each measurement of the(back-) scatter of the (map of) predetermined excitation (for details onthe “same” predetermined excitation, please see below).

In another exemplary embodiment of the invention, the cocoon PUF may beused to reconstruct a two-factor secret from Helper-Data. If the cocoonPUF is changed, the original, correct secret cannot be recovered fromthe Helper-Data any more. This procedure will be outlined in thefollowing in more detail with respect to FIG. 13 and FIG. 14.

As depicted in FIG. 13, the Helper-Data generation process isexemplified, which is to be performed with the un-tampered semiconductormodule. The Helper-Data generation function provided in the hardwaremodule is provided with a secret and uses the PUF data obtained from thecocoon PUF by the measurement unit (collecting the sensors' measuredvalues of the predetermined excitation's (back-) scatter) to generateHelper-Data. The secret is thus split into two parts (one exemplary wayto achieve that is one aspect of the Vemam-Chiffre by Gilbert Vernam),the PUF data and the corresponding Helper-Data. The Helper-Data can beleft un-protected and may be stored in a CASTORs internal or externalnon-volatile memory without any further protection. In order to recoverthe secret, a “secret generation function” is used, which is inversingthe operation of the Helper-Data generating function, given correct PUFdata and Helper-Data are used. Hence, the Helper-Data is loaded from theCASTORs internal or external storage and the secret generation functionobtains the latest PUF data from the measurement unit (collecting thesensors measured (map of) values of latest measurement of the (back-)scatter of the (map of) predetermined excitation) to generate a secret.As long as the PUF data is the same PUF data previously used in theHelper-Data generation process and the Helper-Data is the Helper-Datagenerated in the previous Helper-Data generation process, the correctsecret is reconstructed, otherwise another, however incorrect, “secret”is obtained.

Using this mechanism for tamper protection, some essential data requiredby the firmware to operate the hardware module could be input to theHelper-Data generation function upon setup or initialization of thehardware module, and only the Helper-Data thus obtained is stored in thehardware module (or an external memory). Hence, in order to operate thehardware module, the tamper-protected semiconductor module requires thecorrect secret (i.e. the essential data). Accordingly, the hardwaremodule might generate a measured digital fingerprint of thetamper-protected semiconductor module from the (map of) measured valuesof (back-) scatter of the excitation provided by the sensors, receivethe Helper-Data from memory, and generate a secret based on the measureddigital fingerprint and the Helper-Data (a two-factor secret), asdepicted in FIG. 14. In case the measured digital fingerprint is notcorresponding to the “correct” (previously measured and digitized)fingerprint of the un-tampered tamper-protected semiconductor module,the generated (recombined two-factor) secret is incorrect. This means:It is not corresponding to the “correct” secret that had been obtainedfrom the generation of the secret using the Helper-Data and the“correct” fingerprint of the un-tampered tamper-protected semiconductormodule.

Hence, in this case the hardware module won't be operable as theessential data won't be correct. Furthermore, the generation of anincorrect secret does also imply, the unique (e.g. deflection)properties inside the cocoon must have been changed somehow (i.e. thePUF fingerprint function of the tamper-protected semiconductor modulehas also changed), which in turn indicates an attempt to access theinside of the cocoon (respectively, tamper-protected semiconductormodule), thus flagging a tampering attempt.

Electromagnetic (Radio) Fingerprinting

One possible way to produce a fingerprint is as follows: The cocooninteracts with electromagnetic excitations used for the intensitymeasurement based on applied radio frequency (RF). For example, in oneembodiment of the invention, methods of (near-field) spectroscopy incombination with a matrix of electrodes are used to allow themeasurement of local individual transmission behavior patterns, causedby physical characteristics. Such physical characteristic is for exampledensity. This is influenced by the gap-distributed particle materials(like intraformational conglomerates and components in the cocoon) dueto their characteristic electromagnetic resonances and absorptionsimpacts on the field lines.

In one example implementation an emitter-/sensor phalanx is used totransmit pulses of radio-/microwaves which bounce off and diffuse intothe cocoon. The cocoon returns a tiny part of the wave's energy to theemitter-/sensor phalanx. Measurands of data of such “excitation” may beone or more of the following:

-   -   (displacement of) amplitude (=intensity),    -   (displacement of) frequency,    -   (displacement of) phase and the    -   (displacement of) signal propagation delay.

The quality of the measurands may cover one or more of the following:

-   -   (grade of) dissipation (δ, degree of the lost intensity),    -   (grade of) absorptivity (α, degree of the absorbed intensity),    -   (grade of) transmittance (τ, degree of the leaked intensity) and        the    -   (grade of) reflection (ρ, degree of the reflected intensity)        (where relation between these physical values: ρ+α=1, ρ+τ+δ=1,        α=τ+δ can be assumed).

Atoms and molecules have unique spectra, which can be interpreted toderive information about them. The measured spectra may be used todetermine the composition and physical properties of the functionalgranules of granulates (the functional particles), liquids or gasesfilled bubbles within the potting matrix forming the cocoon.

Transport mechanism of charge carrier and electromagnetic impacts tofunctional materials can be manifold. According to embodiments of theinvention, it is possible to analyze state and sort of the depositedmaterials inside the cocoon through the appearance of material specifictime constants of those (relaxation) effects. Part of these occurringeffects can be reduced to phenomena of the atomic electron shell, toatomic or dipole moments, to (rare) electrochemical charge coupledeffects, to interface boundary layer charges and many other effects.

In practice, (part of) this can be used for the same organic resin andproduction process that is employed in manufacturing of the chip housing(as established in the semiconductor industry) and is well suited ashost matrix to incorporate a dispersed mixture ranging from nano- tomicro particles with a unique combination of (different) particle sizes,(different) shapes, (different) distributions, (different) fillingfactors (volume fraction) and (different) physical properties of theparticles.

Generally, in composite materials, homogeneous dispersion of theparticles is always a challenge because the particles tend toagglomerate. For the present application in a cocoon, however, somedegree of heterogeneity may be beneficial. The degree of heterogeneityin the local distribution of the particles tailors the functionalizationof the particles covering the whole range from non-functionalizedparticles, which tend to agglomerate, to completely functionalizedparticles which can be dispersed homogeneously.

Among the different particles with different physical properties theconductivity of the particle material may be an important quantity. Forexample, large effects may result from using metallic particles. In someexemplary implementation of the cocoon, carbon black or TiO2 can be usedas filler, although metallic particles may allow for a much bettercontrol of the properties to influence fields (lines). Moreover, someimplementations of the cocoon may also use the alloy composition as anadditional adjustable parameter, which is especially interesting formagnetic particles (see below).

Incorporated semiconducting particles into the matrix can take advantageof the resulting nonlinear features of Schottky junctions. For example,fullerenes (i.e. molecules composed entirely of carbon, in the form of ahollow sphere, ellipsoid, or tube, called buckyballs in its sphericalform or Carbon-Nano-Tubes (CNTs) in their cylindrical form), which aretypically produced as a mixture of metallic and semiconducting tubes,offer the possibility to form many Schottky junctions in a random wayduring dispersion.

In other exemplary implementations, ferromagnetic particles may becomprised in the cocoon, because they can be designed to exhibit aferromagnetic resonance frequency in the interesting frequency range.Thus it may be possible to obtain a random but unique distribution ofresonances by using ferromagnetic particles of different size and shapeand, if metallic particles are used, different alloy composition. Viathe particle filling factor, eddy current damping is another means ofgeneration of additional information within the cocoon material. Sincethe chip housings may for example have a thickness in the mm range (thismay be relaxed to even sub-mm range due to the integration requirementsfor lamination and RFID applications) the ferromagnetic resonances andthe eddy current damping can also be used to avoid an extra shieldinglayer around the cocoon. This is also an exemplary way (as alternativeor in combination with special external shielding materials) to reducethe problem of GHz frequency crosstalk between different chips.

The dielectric properties and their spatial variation can be tailored byadding particles with a very high permittivity into the particle mixturecausing radio-wave resonance and Mie scattering. Possible materials areBaTiO3 and YIG, for instance.

The relevant physical properties may depend on the frequency range ofthe electromagnetic radiation which is used for the secret generation,because the transmission (due to dielectrics, magnetics and conductors),reflectance (due to scattering and transmission) on our 3D nano- and/ormicrostructures causing propagation, dispersion and the absorption, aswell as memory effects (e.g., due to capacitive coupling) depend on thefrequency range at hand. Reflection and refraction of RF near-fieldwaves at the boundary of two media can produce hard-to-predict effects:The modeling can be done by using the Maxwell equations or thegeneralized Ewald-Oseen extinction theorem. The exemplary frequencyrange of up to 10 GHz was identified to be as most suitable. In thisfrequency range, available materials allow tailoring physical propertiesover a broad range. The crucial properties are the frequency dependentreal and imaginary part of the permittivity ε(ω) and the permeabilityμ(ω) and the frequency dependent conductivity σ(ω). The imaginary partsof ε(ω) and μ(ω) represent the losses and are closely related to σ(ω).Nonlinear effects can be explored for generating additional information,in particular, by using Schottky junctions.

There are various possibilities how to provide the one or more emitterson the hardware module. For example, one possibility is to use(non-specific) circuitry and/or integrated circuits of the hardwaremodule (e.g. semiconductor elements and/orMicro-Electro-Mechanical-Systems (MEMS)) as emitters. Hence, individualcircuit components, wires or some chip areas of the hardware module mayhave properties of an oscillator/clock generator/radiator and may thusbe used as emitters for emitting the predetermined electromagneticexcitation.

In another exemplary embodiment of the invention a set of (for instance)electrodes is used to excite (stray) fields within the interior of thecocoon. The fan-out of these electrodes may be for example achievedthrough an internal switch matrix, but may also be realized via portconnectors. The cocoon interior could for example form a cavity. Thevarious electrodes could be coupled through (for instance) evanescentfields or resonances, which may be governed by geometrical properties aswell as potentially inhomogeneous material inclusions in a uniquemanner.

By exciting pairs or groups of input and output electrodes, variousfield patterns and thus surface current distributions can be obtained.The position of the electrodes and therefore the field distributions arechosen in a way, that, wherever the shielding (or the cocoon itself) isbroken or deliberately penetrated from the outside, the surface currentsand thus the internal fields as well as the coupling of the electrodeswill be distorted. The distortion will be observed at or can be derivedfrom the port signals.

In order to ensure the emission of the predetermined excitation, thetamper-protected semiconductor module operates the emitters atpredetermined operation conditions during the period in time the sensorsare to measure the (back-) scatter of the excitation. To establish suchoperation conditions, the tamper-protected semiconductor module may(partially) deactivate the (integrated) circuit parts of the hardwaremodule during a measurement cycle in which the one or more sensors sensethe (back-) scatter of the predetermined excitation. For instance, such(partly) deactivation may be a deep power down to minimize theinterference of the (integrated) circuit parts of the hardware modulewith the (back-) scatter of the predetermined excitation.

With respect to the electromagnetic signal exchange nature, it isadditional possible to “check the channel” before executing thereconstruction process revealing the secret. Such a Physical CarrierSensing (PCS) of the environment through passive scanning for latercollision avoidance attitude may be useful to optimize the procedure.

Alternatively or in addition, the circuitry use and/or integratedcircuits of the hardware modules emitter, oscillators/radiators may beprovided on the hardware modules for the purpose of emitting thepredetermined excitation. The entirety of the oscillators/radiatorstogether (and optionally in combination with the emitter(s) formed bycircuitry and/or integrated circuits of the hardware module) emits thepredetermined excitation.

The concept of the cocoon PUF is, however, not limited to the use ofelectromagnetic excitations. The emitter(s) may provide anelectromagnetic excitation (e.g. electromagnetic fields, X-ray, etc.),an optical excitation (e.g. generated by LED or LASER elements) andacoustic excitation (e.g. infra- or ultrasonic), or combinations thereof(please see below). Accordingly, the sensors that sense the (back-)scatter of the excitation are—dependent on the type of theexcitation—electromagnetic sensors, electric field sensors, magneticfield sensors, acoustic sensors, optical sensors or other radiationsensors. The sensors and/or the emitters may be implemented asMicro-Electro-Mechanical-Systems (MEMS) and/orNano-Electro-Mechanical-Systems (NEMS).

Optical (Light) Fingerprinting

Optical imaging is a technique and process used to create images of theenvironment for further processing purposes (physical procedures seekingto reveal, analyze or examine information for an “initial patternlearning” or “recognition match with already known patterns”). Althoughimaging physical structures of interest can be performed for analyzingreasons, such procedures are not usually referred to as picture imaging,but rather could be a part of cocoon fingerprinting. As a discipline andin its widest sense it is part of general imaging and incorporatesinvestigative radiological sciences, thermography, and photography.Measurement and recording techniques which are not primarily designed toproduce images, but which produce data susceptible to be represented asmaps (i.e. containing positional information) can be seen as forms andmeans of fingerprinting imaging that may also be used to generate afingerprint of a given semiconductor module including a cocoon.

An example for such optical fingerprinting that may be used also forgenerating fingerprints of a semiconductor is an optical radar,particularly a laser radar (Light Detection And Ranging (LIDAR)) used torecord range images (the name for a collection of techniques which areused to produce a 2D image showing the distance to points in a scenefrom a specific point, normally associated with some type of sensordevice) or reflection images (same constructed from the reflection andon the formation of light). It consists of a light source (serving as anemitter of the excitation), a light receiver (serving as a sensor) and aprocessing unit. The light source and the light receiver can be e.g.constructed as diode rows which operate in conjunction with one or moreimaging optics in a manner a light pulse from a particular transmitterdiode is imaged on a particular receiver diode after reflection in aparticular scanning field of the objective lens. For fingerprinting asemiconductor module, one or more light sources and one or more lightreceivers could be provided on the hardware module and the scanningfield may be defined within the inside of the cocoon encapsulating thehardware module.

Acoustic (Sonic) Fingerprinting

An acoustic wave is the propagation of oscillations of molecules orpoint masses. The sonic speed is up to pressure, temperature, materialproperties, frequency and amplitude. At every place the contemporaneousmeasurement of time-dependent physical values like the elongation ofatoms/molecules from their middle position, speed, pressure,temperature, and density is possible. This leads to the calculation ofsound pressure, energy density, sound power, sound intensity, and soundpressure level, usable for a fingerprint generation process. An examplefor such principle is the navigation of bats through ultra/supersonics,allowing them the acoustic classification of landmarks andidentification of objects. Another example of sonic fingerprinting isfound in the field of underwater acoustics: The study ofsound-propagation in water and the interaction of the mechanical wavesthat constitute sound with the water and its boundaries. This leads thento a number of other fields of acoustic study, including sonar,transduction, acoustic signal processing, acoustical oceanography,bioacoustics, and physical acoustics. All these areas use the physics ofacoustic waves for fingerprinting purposes in one or another way and theprinciples may also be used for generating an acoustic (sonic)fingerprint of a semiconductor module. For example, an acoustic (sonic)excitation can be emitted by the emitters on the hardware module and thesensors measure (or a processing unit determines from measurement valuesof the sensor(s)) one or more of the parameters sound pressure, energydensity, sound power, sound intensity, and sound pressure level, whichare used to defined a fingerprint of the semiconductor module is somepredetermined way.

Digital Geometry Processing (DGP) Fingerprinting

Digital geometry processing is used to generate a three-dimensionalimage of the inside of an object and its detailed internal structuresfrom a large series of two-dimensional images. Known examples areComputer Tomography (CT) scanning systems and Magnetic ResonanceImaging/Tomography (MRI/MRT) systems. Both produce a volume of datawhich can be manipulated through a process known as “windowing” in orderto analyze various structures based on their ability to block theradiation. Although most common in medicine, they are also used in otherfields, such as nondestructive materials testing. Another example isarchaeological use. In this invention, these mechanisms can be used togenerate a PUF fingerprint inside the cocoon (shielding) of thesemiconductor module.

Shielding Methods

The shielding, of course, also depends on the type of the excitation bythe emitter(s). Hence, the shielding can be an electric, magnetic,electromagnetic, acoustic, optical shielding or a combination thereof(as descript above). The mechanism of action of a shielding is acombination of reflecting and absorbing/attenuating an excitation, sothat only very little excitation would penetrate the cocoon (with theshielding).

The intended effect of a shielding comprises two principle strategies:

-   -   preventing undesirable emittance of the excitation to the outer        world beyond the limits of the cocoon, to minimize EMI and        insofar protecting the environment from a pollution of the        excitation, and    -   preventing a “mixture” of the self-generated “predetermined”        excitation with an excitation coming from the outside of the        tamper-protected hardware. This ensures the sensors measure of        only the (back-) scatter of the predetermined excitation within        the limits of the cocoon (respectively, the shielding provided        therein).        Different types of shielding are to be applied depending on the        type of the excitation by the emitter(s).        Electromagnetic (Radio) Shields

If electromagnetic excitation is used, in one embodiment of theinvention, the shielding should be an electromagnetic shielding toprovide a Faraday cage within itself. For example, the shieldingmaterial may be or comprise a metal or another material with highmagnetic permeability and high electrical conductivity. Thiselectromagnetic shield may shield anything inside its cocoon, theshielding from all electromagnetic excitations, or at leastelectromagnetic excitations up to frequencies in the GHz or even THzrange. Hence, the permeability of the shielding material should beselected accordingly.

In one embodiment of the invention, the magnetic permeability of thematerial of the electromagnetic shielding is as high as possible. In anexemplary embodiment of the invention, the shielding has high magneticpermeability, such as for example permalloy (e.g. a Nickel-Iron magneticalloy). Furthermore, in some applications low coercivity, near zeromagnetostriction, and significant anisotropic magneto-resistance of theshielding material may be desirable. High magnetic permeability forexample means permeability μ_(r) higher than approx. 5,000 H/m.

The electrical conductivity (the specific conductance is the reciprocalquantity, and measures a material's ability to conduct an electriccurrent) of the material of the electromagnetic shielding is as high aspossible. In an exemplary embodiment of the invention, the shielding hashigh electrical conductivity, such as a metal (or a combination ofmetals) like aluminum, gold, copper or silver for example. Highelectrical conductivity for example means a conductivity σ (commonlyrepresented by the Greek letter σ, but also the Greek letters κ and γare occasionally used) higher than approx. 106 S/m.

In an exemplary embodiment of the invention, the shield of the cocoonmay cover the cocoon's outer surface, and may be only broken at theconnection ports.

Optical (Light) Shields

An optical shielding may for example comprise one or more surfacereflectors or could be implemented as a surface reflector that covers atleast a portion of the hardware module. For example, in one exemplaryimplementation, the surface reflector(s) could be made of metal (e.g.aluminum, silver or the like) in form of a coating e.g. on the innerand/or outer surface of the cocoon. Another exemplary realization of anoptical shielding is to use the principle of optical fibres. The lightis “waveguided” on the top of the cocoon on the hardware module by theshielding—acting as “light pipe”—using a process of total reflectionbetween two dielectric materials of the shielding with differentrefractive indices. Another way is the more or less high absorptionknown from black bodies.

Acoustic (Sonic) Shields

An exemplary known way to shield acoustic excitation is the anechoicroom that can be for example provided inside the cocoon (more precisely,the shielding) of a semiconductor module. An anechoic chamber isdesigned to suppress the acoustic echo-waves and isolates from theacoustic noise present in the external environment.

Another way is the usage of Active Noise Reduction (ANR) inside thecocoon of the semiconductor module in cases where the structure of thenoise is known (e.g. by using internal and/or external microphones).Noise-cancelling may be for example realized by emitting a sound wavewith the same amplitude but with inverted phase (also known asanti-phase) to the original sound. The waves combine to form a new wave,in a process called interference, and effectively cancel each otherout—an effect which is called phase cancellation.

Another exemplary implementation of an acoustic shielding is providedusing shielding material able to comprise or consist of permanentlyelastic compound structured foamed material, which could be providedinside the cocoon of the semiconductor module.

Magnetic Shields

In another embodiment of the invention, a magnetic shielding can beprovided. The conductivity/permeability of the shielding material shouldbe as high as possible. In an exemplary embodiment of the invention, theshielding material comprises or is a material that has a highpermeability, such as permalloy for example (e.g. a Nickel-Iron magneticalloy).

Furthermore, in some applications it may be desirable that the shieldingmaterial furthermore has low coercivity, close to zeromagneto-striction, and significant anisotropic magneto-resistance. Highpermeability may for example mean permeability/higher than 50,000 H/m orat least 5,000 H/m.

Electric Shields

Another possibility is to use an electric shielding. The electric shieldis reducing the electrostatic field in a space by blocking the fieldwith barriers made of conductive materials. The same precautionarymeasures applied against the electromagnetic shielding are also suitablefor blocking this type of field.

Creation, Generation and Manufacturing Methods

Production of the Shielding

The shielding can be produced by using one or more of the followingtechnical processes for producing different layers of shielding materialon the inside and/or outside of the cocoon surface:

-   -   sintering one or more shielding materials,    -   sputtering one or more shielding materials,    -   spraying, spritzing or bubbling one or more shielding materials        (e.g. using High-Volume-/Low-Pressure plants (HVLP)),    -   coating and/or plating one or more shielding materials (e.g.        electric conductive (finishing) paint/lacquers containing/made        of epoxy, polyurethane or acrylic with expletive particles like        silver, silver plated copper, pure copper or nickel),    -   immersing in one or more baths of fluids comprising the        shielding material,    -   evaporating, optional in a vacuum, of one or more shielding        materials (e.g. Physical Vapor Deposition (PVD)),    -   electroless plating one or more shielding materials (e.g. one        layer/sheet/lamina e.g. of pure copper, followed by another        layer/sheet/lamina e.g. of nickel-phosphorus alloy),    -   electrolysis using one or more same shielding materials.        Creation of the Fingerprint

Regarding the predetermined excitation, there are also variouspossibilities how to generate such predetermined excitation. Toreproduce the “fingerprint” (of the un-tampered hardware) thepredetermined excitation should be the same in each measurement cycle inwhich the sensors sense their measured values that are used to determinethe actual “just measured” fingerprint. In practice, it may be difficultto achieve identical excitations (for example, due to temperaturevariations, drift effects of physical material properties in time, thefact of a noisy measurement, fuzzy sensors, etc.). Hence, the sameexcitations should not be interpreted as 100% identical excitations;however, the variations may be compensated by the fingerprint generationprocess (e.g. using error detection and correction mechanism, e.g. ECC,EDC and CRC) of the fingerprint from the measured values provided by thesensors and/or countermeasures may keep variations of the excitation inan acceptable range not critical to the uniqueness and stability of thegenerated fingerprint. In one exemplary embodiment, the cocoon'sinfluence the (back-) scatter of the excitation to be sensed by thesensors is insensible to change of temperature (at least within adefinable range or desirable range of temperatures).

There may be a tradeoff in terms of a mutual influence between theimplemented countermeasures keeping variations of the excitation in anacceptable range not critical to the uniqueness and stability of thegenerated fingerprint (effectively, such countermeasures may reduce theamount of usable digital data for the fingerprint) and the generationprocess of the fingerprint from the measured values provided by thesensors (i.e. the measured values so to say forms the data basis fordetermining the fingerprint): On the one hand, the generation process ofthe fingerprint should not reduce the data basis for the generation ofthe fingerprint such that it loses its uniqueness; on the other hand,countermeasures keeping variations of the excitation in an acceptablerange may aid to minimize the reduction of the data basis for thefingerprint, but may be expensive to implement. However, by providing asufficiently large data basis, i.e. sufficient number of measured valuesby the sensors for the countermeasures to keep variations of theexcitation in an acceptable range may be one solution to solve thistradeoff.

To generate a sufficient data base (i.e. a sufficient number of measuredvalues (respectively “bits”) once the measured values are quantized)that can be used for the generation of the unique fingerprint of thetamper-protected semiconductor module, different possibilities to designthe predetermined excitation of the one or more emitters are possible.For example, one possibility may be that the predetermined excitation isa constant predetermined excitation, i.e. not varying in time.Alternatively, the predetermined excitation could be also an excitationthat is varying in a predetermined manner—which implies that the sensorsshould measure the measurand multiple times within a measurement period.The variation of the excitation could be for example implemented by thehardware module comprising an excitation controller, also referenced asFuzzy Extractor (as shown in FIG. 21), that controls the emittersindividually or in combination to produce the predetermined excitationvarying in time. For example, different sets of emitters, alsoreferenced as part of the Emitter-/Sensor Phalanx (as shown in FIG. 21)are used in a given sequence in time to generate respective excitations(i.e. a predetermined sequence of individual excitations) that form thepredetermined excitation.

In one exemplary implementation of the invention, the measurementprocess uses the feature of localization (pinpointing) to increase theavailable database of measurands.

In a further embodiment, the sensors to provide a measured value of themeasurand that is to be part of the data basis for generating thefingerprint could be controlled by the hardware module. For example, thehardware module may trigger/query different sets of sensors at differenttimes of a measurement cycle to receive their measurement results. Thiscontrol of the sensors may of course be readily combined with thecontrol of the emitters described in the preceding paragraph.

For the purpose of controlling emitters and sensors and optionally ofdeactivating the (integrated) circuit parts of the hardware moduleduring a measurement cycle, a measurement unit may be provided on or bythe hardware module (an exemplary implementation is shown on a blockdiagram within FIG. 21: “Key Control”). This management unit may beconsidered an IC of the hardware module or may be implemented in itsfirmware. The management unit transmits and/or receives commands and/ordata packets from the other integrated circuit parts of the hardwaremodule, e.g. the sensors and emitters to control their emission,respectively receive the measured values. Further, the measurement unitmay coordinate the one or more sensors and the one or more emitters in ameasurement cycle.

Please note, the cocoon may be the housing of the hardware module or mayitself be included in a housing together with the hardware module.Likewise, the shielding of the tamper-protected semiconductor modulecould be part of the cocoon or may enclose same. In case a “separate”housing is provided the cocoon could be provided in form of a malleablemass between the housing and the hardware module.

Manufacturing of a Tamper-Protected Semiconductor Module with Cocoon PUF

Another aspect of the invention is the provision of a manufacturingprocess for a tamper-protected semiconductor module with a cocoon PUF.According to an embodiment of the invention the manufacturing process ofa tamper-protected semiconductor module with a cocoon PUF comprises theprovision of a hardware module equipped with one or more emitters foremitting a predetermined excitation that can be measured as a physicalmeasurand, and one or more sensors for sensing the (back-) scatter ofthe excitation in form of the physical measurand in a contactless and/orcontact-based manner. Further, as another step of the manufacturingprocess, the hardware module is housed in a cocoon to realize thetamper-protected semiconductor module, wherein the cocoon influences the(back-) scatter of the excitation sensed by the sensors in a way uniqueto the tamper-protected semiconductor module.

Housing the hardware module in the cocoon comprises enclosing thehardware module in a potting material, and encapsulating the hardwaremodule enclosed by the potting material with a shielding to preventinterference with the predetermined excitation from the outside of thetamper-protected semiconductor module. As noted above, the shielding maybe but is not necessarily part of the cocoon.

Optionally, the shielding may be subject to thermal treatment. Furtheroptionally, the manufacturing method provides a passivation (e.g.electrical isolation) to the hardware module.

The hardware module may be manufactured in a standard wafer process, forexample a silicon semiconductor process.

In the following, some exemplary methods for producing cocoons arepresented. The invention is however not limited to these methods, butalso other suitable methods may be used. A cocoon may also be viewed assensing element driven by stimuli excitations and reporting-backmeasurands (measured data). As a result, the fingerprint of the cocooncan be obtained. This is shown in principle in the FIG. 20.

There are many ways to manufacture cocoons with attributes ascontemplated by this patent application. Different possibilities tobuild a cocoon are described in the “cocoon design methods” set outbelow. Cocoons can be produced using only one of the manufacturingconcepts outlined below, i.e. applying only one of them as single cocoondesign method, but it is also possible to use a combination of more thanone of the manufacturing concepts outlined below, i.e. to combine cocoondesign methods set out below to manufacture cocoons.

Cocoon Design Method: Cavity Resonators

The mechanical arrangement of the cocoon and its unique materialproperties may sometimes lead to individual cavity properties. These arediscoverable through a measurement of resonance behavior over theapplied frequency range in form of pronounced maxima and minima (modes)field distributions.

In one exemplary implementation, the cocoon excitation will be carriedout with wideband signals, having spectra from e.g. some kHz or MHz upto several GHz (kind of pulsed Ultra-Wide-Band (UWB) signals), inanother exemplary implementation, this could be extended to reach evenTHz (it is a matter of costs and the design restrictions due to theproduction technology someone is willing to take).

One possible choice is the usage of Gaussian monocycles, which can beproduced in a cheap manner, but also other time-domain waveforms arepossible. The usable bandwidth of the signals may be even extended bypartially non-linear properties of the filling material (fieldamplitude-dependence of the constitutive material parameters, e.g.through internal Schottky-contacts formed by metal and semiconductorinclusions (fullerenes as CNTs)).

As already noted, electrodes are an example for means to couple theexcitation and responding reaction as impulse answer or transferfunction between the cocoon and the cocoon PUF circuitry. For everychosen electrode combination (responsible for the excitation and themeasurement), the (wideband) output signal may be recorded in anexemplary implementation by sequential (sub) sampling (subsampling isone of the prevalent method of choice for periodic signals). This canallow for extremely high effective sampling rates utilizing slow andcheap analogue-to-digital converters. From the entity of the variousrecorded signals, a unique fingerprint can then be derived bysophisticated signal processing (Fuzzy Extractor).

Cocoon Design Method: Self-Organization and StructureFormation/Texturing

The effect of structure formation and texturing of self-organizationlike crystal growth (epitaxy) for example on boundary layers and thinfilms (e.g. Frank-van-der-Merwe-growth, Stranski-Krastanov-growth orVolmer-Weber-growth) generates very many-body (particles) systems from ahuge number of very small objects. A well known example for crystalgrowth is the growth of ice crystals.

The basic mechanisms as triggering events for interaction of thesedifferent types of self-organization are van-der-Waals-forces,Hydrogen-bonds, electrostatic-forces and the steric-effects. They form acomplex balance between forces, figures, distances and orientations.This leads to the process of crystallization of solid state bodies.Properties of surfaces are also decision-making factors, which can beused systematical in the task of “Crystal Engineering”. Such acontrolled crystal growth leads in a first step into the composition ofnanocrystals in a microemulsion, ending up in a 3D crystal cobwebforming up and structuring the cocoon material.

The aimed doping of guest-atoms into grain boundary voids andimperfections in the laminate of the presented cocoon leads into amacroscopic dispersion measurable differences of qualitycharacteristics.

Another exemplary implementation for the crystallization of polymerscaused by the spaghetti syndrome, where form and size constrain theregular alignment of crystal structures resulting in crystal errors, isthe use of nano-structure materials consisting of block-co-polymer ofdifferent—immiscible—single components results into microphaseseparation, where a combination of multiple different blocks build upmore and more complex macro-structures. This results into an individualcocoon. In another exemplary implementation, nano-structure materialscan be combined with “hard-” and “soft-matter” components of hybridmaterials in sandwich layers, where in each case an organic polymer filmserves as molecular adhesive in a combination with an anorganic layer asbarrier and amplifier stacked one upon the other as another way togenerate a cocoon suitable in the sense of this use-case.

Cocoon Design Method: Mechanical States of Stress/Tenseness

Another exemplary solution to create a cocoon is the building-up of veryhigh stress latency during the state change (from the liquid) into thesetting phase of the cocoon material. This could be shaped as strongbonding of the molecules. If a cocoon constructed like this is injuredby trying an invasive penetration (e.g. by applying a hole drillinginstrument (see also chapter “Reverse Engineering of a Cocoon PUF”)),the cocoon will be teared apart into fragments.

The cocoon may be thermally pre-tensioned. This may be achieved forexample by a quick cooling down (chilling) procedure for the outersurface of the cocoon after the heat build-up.

Such a treatment creates a compressional stress on one surface of thecocoon material (due to quick cooling). Further, the in the core up tothe other inside surface of the cocoon there may be a compensationaltensile stress profile (due to slow cooling). This stabilizes on the onehand the nominal bending strength, but affected on the other hand a moreor less completely torn down cocoon smashed to smithereens in case ofmechanical interferences.

Cocoon Design Method: Small Spherules

Colloid polymer dispersions of particles (exempli gratia latex) createsuperior structures during the state transfer from the solvent phaseinto the setting phase of a pre-productional cocoon material componentpart. In a process of further complexation of polyelectrolytes (symplex)microcapsules are emerging from it. This process is then steered up to amoment in time, where these microcapsules (filled with liquidelectrolytes) are big enough to fulfill their duty: as part of thecocoon potting material an injury of their diaphragm allows for aleakage of their liquid into the embedding and surrounding area. Inanother exemplary implementation of the invention, the diversity ofmicro-capsules will be extended to allow different electrolytes and/ordifferent capsule sizes. The emergence of the liquid causes asignificant change of measurable parameters building the cocoonfingerprint.

Cocoon Design Method: Under- or Overpressure

As already noted, the cocoon may have an air gap or cavity between theprotected core part in its center (mostly referenced as tamper-protectedhardware module) and the inside delimiter of the cocoon itself. This airgap or cavity may be for example filled with a sort ofpressure-sensitive (semi-rigid) foam, e.g. using foam of fullerenesand/or of polyurethane elastomers. This foam is endowed by usingsubstances influencing the physical effect sensing the (back-) scatterof the excitation in a form that the measured fingerprint depends on thepressure. The filling could be for example done in an environmentworking in under- or overpressure conditions, both suitable for thisdesign method. The exact pressure may be randomly chosen within a windowof allowed minimum and maximum over- or underpressure in comparison tothe “normal pressure” at production time.

After the manufacturing of the cocoon, the protected device will bepressure-sealed closed, before leaving the production facility with theatmospheric under- or overpressure.

The foam is not changing its constitution (of granularity) as long asthe pressure is not changes. This applies also to the fingerprint of theprotected device: it will not change (up from the time of birth of thedevice) as long as the pressure is stable.

In case of any significant pressure change (as for example introduced bya hull breach of the cocoon), the foam structure (collapses and) changesirreversible too. As a result, the fingerprint of the device cannevermore be reconstructed, the secret is gone.

Exemplary Application of the Cocoon PUF: CASTOR/Nano-Safe

One exemplary use for the electromagnetic (radio) fingerprinting type ofthe cocoon PUF is the CASTOR, as exemplarily shown in FIG. 21 as a blockdiagram. The block “Cocoon-PUF” covers the disclosed cocoon in a logicalformal functional way. The cocoon PUF is exemplified as a “box” becausethe cocoon itself is not part of the structure of the one or moresemiconductor module(s). The box indicated as “Third ‘IP’ Party'sCircuitry using the Secret” comprises the user-based tamper-protectedsemiconductor module(s) itself. The Helper-Data is the content of anon-volatile memory, which is insensible under security aspects andinsofar not in the need of being protected. It can be situated in- oroutside of the security perimeter of the CASTOR. The remainingfunctional “blocks” are explained in the following.

The Emitter-/Sensor Phalanx

Due to the electromagnetic nature of the coupling principle between thePUF circuitry on the die and the physical cocoon an emitter/sensorphalanx is used to emanate and receive the electromagnetic waves. Due tomechanical space restrictions on the die, one exemplary possible way isthe usage of a plurality of electrodes forming a matrix located on thesurface of the die. Another exemplary implementation—with can becombined with the before-mentioned matrix of electrodes—is the (partly)use of the connection terminals of the chip (as shown under FIG. 11 andFIG. 22).

One potential obstacle may be the geometric small semiconductorstructures compared to the wavelength: Using an exemplarily excitationfrequency range of 1 up to 10 GHz, this corresponds to a wavelength of30 cm to 3 cm. Another higher range of 10 to 100 GHz corresponds to 3 cmdown to 3 mm and even in a very high range between 100 GHz and severalTHz the wavelength is less than 3 mm. One effect of this is that thesmaller the electrodes are compared to the wavelength, the smaller theradiation resistance. The problem is that since energy losses increasewith decreasing radiation resistance, the energy radiated by theelectrodes will drop with reduction of their size. Electric field lineshardly emanate from (extremely) small antennas or aerial points. Theyonly slightly detach and travel into space and thus contribute onlylittle to the radiation of energy. They do not tear up, but migrate backto the generator, i.e. the respective electrode. Consequently theymerely represent reactive power with a net flux of zero. Due to thisnear-field measuring, electrodes are an exemplary choice to emanate andmeasure the excitation for media different from free space (usually airor vacuity).

The Fuzzy Extractor

The main task of the Fuzzy Extractor in FIG. 21 is to extract thefingerprint information out of the cocoon PUF. This is not a simplequantifier task like an analog to digital converter scheme. Theinformation of a PUF is blurred in the ideal case all over the medium.The Fuzzy Extractor is therefore able to get the specific weasel averageinformation content of the cocoon and have to convert it to sharp data.This is why such a behavior is called fuzzy logic.

For the extraction of the fingerprint from the cocoon PUF, methods forquantifying (spatial) pattern into digital data can be used. Oneexemplary implementation for the digital signal processing can be theFast Fourier Transformation (FFT) as stand-alone method or incombination with the Generic Deterministic Model (GDM) using theVolterra equations or as another exemplary implementation the GenericStatistical Model (GSM) applying the Hidden Markov Model (HMM).

The Fuzzy Extractor may be a combination of an error correction unit(for example by using BCH codes) disposing off the noise and a hashfunction to guarantee a uniform distribution of the bits. The definitionof noise includes environmental influences like heat, dampness, signs ofwear and other influences changing the cocoon PUF's material properties.

The Key Control

The Key Control unit as shown in FIG. 21 can be considered a controllerof the cocoon PUF circuitry, particularly with regards to handling itsfunctionality. The Key Control unit contains intelligence to handle thealgorithms and local intelligence for the other (sub) functionality,such as for example the signal extraction and processing. The centraltasks of the Key Control unit may be one or more of the following:

-   -   the control of the cocoon PUF-based security subsystem,    -   enrolling an external offered secret into the system as last        action of production,    -   internal reconstruction of the secret and its delivering over a        simple interface,    -   management of the Helper-Data stored in internal or external        non-volatile memory.        Helper-Data Non-Volatile Memory Entries

During the very first time of a cocoon PUF initialization for theenrollment of the secret, the cocoon PUF makes a characteristicdetermination of the physical cocoon structure. This is calledCompartment Management (CM) and can take several minutes of processing(formatting) time. The respective produced data is stored within theHelper-Data memory.

For instance, a checksum algorithm or hash could be used to produce afingerprint of the secret computed during the enrollment process. Thesame checksum algorithm or hash is then also used for the purpose ofdetecting reconstruction “read” errors that may have been introducedduring the PUF access. The integrity of the reconstructed secret can bechecked at any later time by recalculating the checksum and comparing itwith the stored one. If the checksums match, the secret is valid.

The following items may be to be stored within the Helper-Data memory:

-   -   a-priori knowledge for the Fuzzy Extractor (optional),    -   cocoon Compartment Management data (CM) (optional),    -   cocoon nominal value fixpoints for cover breach detection        (optional),    -   selected bit cell positions or used model function with        parameters (optional),    -   the public part of the secret, i.e. the Helper-Data,    -   a crypto fingerprint of the reconstructed secret (optional),    -   a checksum of Helper-Data content (optional).        Perspective of an ASIC Designer

The CASTOR works from the perspective of an application chip designer inthe following way: Assuming that the protected application needs forexample a cryptographic key to decrypt data or an embedded firmware. Therequired secret (i.e. the key) is not stored in any non-volatile digitalmemory or storage; it is part of the analog PUF information mechanism ofthe cocoon. The key can be requested by “ringing the bell” using aninternal—very simple—interface of the cocoon PUF and will be deliveredwithin short time. The easiest usage model is to request the secret justafter power-on of the chip and store it in a register-based buffer. Moreelaborated utilization will minimize the time the unencrypted secretwill stay in any volatile digital registers or memory-cells.

Detecting of and Responding to Attacks

A further aspect of the invention is to provide a method for detectingan attempt to tamper a tamper-protected semiconductor module equippedwith a cocoon PUF. The hardware module of the tamper-protectedsemiconductor module with the cocoon PUF

-   a) causes the one or more emitters of the hardware module to emit    predetermined excitation, and further-   b) caused the one or more sensors of the hardware module to sense    the (back-) scatter of the predetermined excitation reflected by the    cocoon of the tamper-protected semiconductor module. Further, the    hardware module then-   c) generates a digital “measured fingerprint” from (back-) scatter    of the predetermined excitation sensed by the one or more sensors,    and-   d) verifies the integrity of the cocoon by using the measured    digital fingerprint recorded.

The “measured fingerprint” is denoting the fingerprint derived from themost recently measured values of the (back-) scatter of thepredetermined excitation sensed by the one or more sensors.

The hardware module may perform steps a) to d) not only once, but alsorepeatedly. Hence, an ongoing verification of the integrity of thetamper-protected semiconductor module is possible. Steps a) to d) may befor example repeatedly performed in irregular intervals or on a randombasis.

-   e) In this further step of the measurement process, an interrupt is    produced, in case integrity could not be verified in step d). This    interrupt may cause the hardware module to permanently or    temporarily make itself unusable. In a further embodiment of the    invention, the measurement process further comprises another step-   f) of deactivating integrated circuit parts of the hardware module    during a measurement cycle in which the one or more sensors sense    the (back-) scatter of the predetermined excitation. This may be    helpful to produce the same predetermined excitation in all    measurements.

In a further exemplary embodiment, the method for checking the integrityof the cocoon may further comprise the following steps of

-   g) initializing the hardware module by creating and recording a    fingerprint of the un-tampered semiconductor module,-   h) securing this fingerprint of step g) using a certificate signed    by an Certification Authority, and-   i) storing the certified fingerprint in a semiconductor non-volatile    memory. In one exemplary embodiment, this certificate may be signed    by the Root Certification Authority (RCA) or another Certification    Authority that can be validated against the RCA's certificate    containing the public key part of the RCA's public key-pair.

Another possibility for verifying the integrity of the cocoon is the useof the cocoon PUF as part of a key restoration process (two-factorsecret, also referenced as “Vernam-Chiffre”). This key restorationprocess is used to recreate the key (also referred to as “recombinedsecret”) from Helper-Data using the cocoon PUF fingerprint. TheHelper-Data is generated based on the key using the cocoon PUF of theun-tampered cocoon, respectively, un-tampered (tamper-protected)semiconductor module, which binds the recreation of the key to thisspecific cocoon PUF. If the cocoon PUF is altered due to a tamperingattempt, the key may be temporarily or permanently destroyed, as thecocoon PUF will be temporarily or permanently changed as a result of thetampering attempt, so that the key generation function is unable toreconstruct the correct key (again). Hence, in this exemplaryembodiment, the verification of the integrity of the cocoon is realizedby means of a key restoration process generating a key based onHelper-Data and the sensed (back-) scatter of the predeterminedexcitation.

The generation of the Helper-Data may be, for example, part of aninitialization procedure of the semiconductor module in an un-tamperedcase (e.g. by performing the initialization at a trusted party; atrusted party could be for example a Root Certification Authority, themanufacturer of the tamper-protected semiconductor module, themanufacturer of a device comprising the tamper-protected semiconductormodule, the customer/user, etc.). Therefore, in another embodiment ofthe invention, the method further comprises the steps of g) initializingthe hardware module by creating and recording a fingerprint of theun-tampered semiconductor module, and generating the Helper-Data basedon the key and the sensed (back-) scatter of the predeterminedexcitation. This Helper-Data may then be stored in a non-volatilememory.

Please note that the Helper-Data may not need to be protected, since itis impossible to recreate the secret from the Helper-Data without thecorrect hardware PUF. Hence, the Helper-Data may be stored either in thehardware module or in an external storage of a device or anotherhardware structure that contains the tamper-protected semiconductormodule.

Reverse Engineering of a Cocoon PUF

Cryptographic solutions are more or less sensitive to attacks that areprimarily exploiting ‘physical’ rather than mathematical weaknesses.Hence, these attacks could rely on implementation-mistakes orexploitable environmental properties. Since the existence of the need tosecure a secret, a lot of physical and side-channel attack methods havebeen “invented and launched”.

Indeed, it is a fair assumption that it is impossible to hardenintegrated circuits totally against all forms of attacks; this mightalso prove true for the CASTOR. It is assumed, that—with enough“criminal energy”—it will always be possible to analyze the functionallogic out of any design. The only and single item what can be defendedup to a sensible limit (determined by the mechanisms of economics: Thecosts in money and time for breaking a system must be reasonable higherthan the value of the protected assets and the outlay to protect it) isthe secret protected by the cocoon PUF. The challenge is to destroy thissecret before the attacker can get it.

The attacker has to surmount the cocoon in a way that the secret isstill extractable. If this can be done, the rest is easy: Tapping theinternal interface to the Key Control unit and initiating a “SecretReconstruction” is enough. To prevent such a scenario, the potentiallymost important feature of the cocoon PUF may be its fundamental abilityto protect even in a passive “out of service” de-energized (no supplyvoltage) state: The fact of a cocoon cover fracture is a sufficientcondition for the secret to vanish into thin air.

To make it more difficult for an attacker, according to anotherembodiment of the invention, essential design parts of the semiconductormodule (e.g. one or more selected from the circuitry of the cocoon PUFresponsible to the weighting of the cocoon fingerprint, the conductingpaths/wires between the cocoon PUF circuitry, and the critical “secretusing” parts of the “Third ‘IP’ Party's circuitry using the Secret”) arenot be placed on the surface of the die. For example, the essentialdesign parts may be situated under a metal layer.

Maze of Knossos—Configuration of the FPGA

Another aspect of the invention is the use of a unique configuration ofFPGAs (Field Programmable Gate Arrays) on a chip (after programming theFPGA to fulfill the desired functionality), which allows integritychecking based on the uniqueness of their configuration. FPGAs may beused on chips to provide various customized functionality. The uniqueconfiguration of the FPGA after its programming can be considered afingerprint of the FPGA itself. For example, a FPGA may be used toimplement a crypto-coprocessor though this aspect of the invention isnot limited to this use.

The Maze of Knossos introduces another element of chip individualizationthrough randomness. It allows a limited change of the soft-corefunctionality even during runtime on a small scale similar to the FPGAfundamental basic approach. The programming is done through sixnon-volatile configuration memory cells in every interconnection point,which are distributed all over the maze array.

The FPGA inspired maze structure consists of many functionalProcessing-Elements (PE) inter-connected by Switch-Boxes (SB) as shownin FIG. 8. The PE elements are scaled on their function and not on thegate level. A type code indicates their functionality. Different valuesmean the PEs provide different functionality. Thousands of different PEtypes could be designed. The distribution of the PEs (with theirintended functionality) on the die changes with every mask revision.

FIG. 9 exemplarily shows the more detailed structure of the PEs. In FIG.9 a PE of type ‘6789’ is exemplified, however this hasn't anyimplications on the PEs' structures. The PEs are optimized and fixed(not programmable) hard macros consisting of a simple control(sequencer) logic, two different logic functions, a small register bank,and a SRAM based distributed memory.

FIG. 10 exemplarily shows the logical design-principle of a Switch-Box(SB). In this example, every SB consists of four inter-connection-pointsacting as routing element with 4×4 rows and columns. Everyinterconnection may for example consist of six pass transistors, likeprogrammable switches (shown in the middle). The gates of the passtransistors are driven by non-volatile configuration memory cells. SBsbehave like inverse fuse based connections: non-programmedinterconnection points represent an open connection (shown on the lefthand side of FIG. 10). After programming the SB connects its neighborsas shown on the right hand side of FIG. 10.

The Maze of Knossos can be used to realize a programmable limitedcryptographic functionality. Due to its structure, there are multipleways to create identical functionality using different configurations ofthe PEs and SBs, comparable to a maze in which more than a singledirection is pointing the way out. This translates to the possibility ofdifferent sets of different programmed PEs and switch-boxes in factleading to the same overall functionality being provided by the FPGA.This means, though they provide the same functionality, each FPGA withina chip can be implemented in a unique way.

The unique selected set of configuration memory cells within a FPGA incombination with uniqueness (introduced by the PUF) thus builds afragile well-balanced system which allows testing the chip-identity. Anycloning of the chip can be detected, if either one of the configurationsof the PEs or SBs is altered, due to the change of its FPGAconfiguration.

For integrity checking of the chip, the unique configuration of its FPGAcan be used in different fashions, similar to the use of a PUF.

In one exemplary implementation, the integrity check based on the FPGAconfiguration (“FPGA fingerprint”) could be chip internal. In this casethe FPGA configuration of the chip is recorded by a trusted party (e.g.RCA) in form of a certificate which is stored in memory inside the chip.By validating the certificate of the original FPGA configuration thechip can assure the FPGA's configuration hasn't been changed.

In this exemplary implementation, the chip could provide some integritychecking function (e.g. as part of the firmware) which checks theintegrity of the chip during bootstrapping. The integrity checkingfunction may read the chip's current FPGA configuration and compares itto the original FPGA's configuration provided in the certificate. Amismatch indicates that this chip is a clone.

The FPGA configuration may also be used to allow an authority to checkthe integrity of a chip. In this case the authority (e.g. host device,the RCA, an institution manufacturing a product containing the chip withthe PUF, etc.) has recorded the original and correct FPGA configurationfor a chip. For integrity checking, the chip is queried by the authorityto obtain its FPGA's configuration. If the FPGA configuration providedby the chip is correct, i.e. if it matches the previously storedoriginal FPGA configuration, the authority may confirm the integrity ofthe chip, or the host device, respectively. Otherwise the authoritymight for example cause a disablement of the chip or its host device,respectively, which could be implemented via a CRL as well.

In one exemplary embodiment, a FPGA is used to implement acrypto-coprocessor in a tamper-protected chip for use in a transactionsystem as described herein. In this exemplary embodiment, the RCA mayrecord the FPGA configuration as part of the CASTOR'scrypto-initialization at the RCA (for more details, see section“Crypto-Initialization of the Tamper-Protected Hardware Module(CASTOR)”). The recorded original FPGA configuration may be used by theRCA to verify integrity of the CASTOR or may also be used by the CASTORchip internally for verification of the integrity as described above.

Furthermore it should be apparent that using the uniqueness of a FPGA'sconfiguration for integrity checking may be readily combined with othersecurity measures for integrity checking, e.g. the use of a PUF on achip. If the RCA's certificates were used for the integrity check, theRCA might provide all relevant information needed to confirm integrityin one certificate.

On-Chip Trusted Time Source

An option for the CASTOR-internal provision of a trusted time could bethe integration of an energy-buffered and trusted real-time source onthe CASTOR chip with a battery. However, as the battery may be difficultto integrate in the chip (even enlarging the waste disposal procedures),in one embodiment of the invention the CASTOR chip is provided with astrong monotonic counter. This counter always reflects the CASTOR'slatest date and time. If date and time information provided by thecounter becomes invalid (e.g. due to a power-loss), a trustedresynchronization with a reliable source is performed (e.g. bycontacting the RCA or other authority able to provide a signed, new, andup-to-date date and time). The resync may be for example performed inthe following events:

-   -   If the CASTOR discerns another CASTOR's date and time as ‘more        recent’. This can be detected e.g. in the Pairing Phase (as for        example described in the priority application PCT/EP2011/001219        of the same applicant, but is not limited to such use), where        the CASTORs include local timestamps into the messages. Please        note, as a measure of security in CASTOR-to-CASTOR        communications only a “move forward” in time is possible but not        a move back into the past.    -   If in CASTOR-to-authority communications an automated update        triggers and resynchronizes the date and time or a push message        with an updated date and time received from trusted authorities,        the CASTOR is provided with a signed ‘set-monotone-counter’        message and takes over this certified date and time, even in        case the system is brought ‘back to the past’.        Crypto-Initialization of the Tamper-Protected Hardware Module        (CASTOR)

Private keys (as private key parts of public-key-pairs or as secretsymmetrical keys) of a CASTOR are managed internally inside the CASTORand are never revealed or propagated to the outer world. Keys (if)stored externally are secured beneath the umbrella encryption (all datathat is passed to the outside of the CASTOR via the CASTORsI/O-interface e.g. the PSSBI discussed above, is protected by using thismethod). All keys are managed by the CASTOR's firmware. They arecreated, used, replaced and destroyed based on a security policy (as forexample described in the priority application PCT/EP2011/001219 of thesame applicant, but the invention is not limited to such use), withoutany user intervention. After the production process of the CASTOR chip,a validation by the RCA takes place with the effect to generate avalidated CASTOR chip. As part of the validation process:

-   -   The CASTOR chip may itself generate its identifying descriptor        item (e.g. serial number) based on cryptographic physical        randomization effects.    -   The CASTOR chip itself may generate a public key-pair used by        the CASTOR for signing to thereby allow verification of the        CASTOR's identity in the transaction system. The private key        part of this signing key-pair is denoted SUDI (Secret Unique        Device Identity), and the public key part        (K_(S:CASTOR:PUDI,pub)) of the signing key-pair is denoted PUDI        (Public Unique Device Identity).    -   The CASTOR chip itself generates its CON (Consistency) public        key-pair for internal hash signing and checking purposes.    -   The RCA signs the PUDI key and provide a cryptographic hashed        version of the signed PUDI key in the CASTOR root certificate        (Type-VI) which is thus individual for each CASTOR (also        referred as “Cert.LicenseID” within the CASTOR root        certificate).

Some keys are truly random number based, self-generated on chip by theCASTOR (please refer also to section “Keys used in the TransactionSystem” as described in the priority application PCT/EP2011/001219 ofthe same applicant, but the invention is not limited to such use) neverleaving the protected environment of the CASTOR and never being exposedto the outer unsafe world. This generation mechanism is also true forthe CASTOR identifier (ID) and the individual en/decryption key for theumbrella protected access via the CASTOR chip's I/O-interface.

Each CASTOR undergoes a validation process at the RCA, as noted above.Without such validation, the CASTOR is useless. This validation at theRCA, also referred to as initialization procedure, may include thefollowing steps:

-   -   An on-chip key and identification generation process is        initiated by an external command through the I/O-interface of        the CASTOR chip. For example, this process can only be done once        in a CASTOR's lifetime (the ID change of a CASTOR is        impossible). During this on-chip key and identification        generation process, the above mentioned keys and objects are        generated—among other things (e.g. the setup of the security        system).    -   The PUDI and the public key part of CON are delivered to the        external world, the RCA. The PUDI may be handed over to the        outside of the CASTOR chip without any restriction in time or        how often this could be done. The public key part of the        consistency key-pair will be delivered to the outside of the        CASTOR chip only once in a lifetime of the physical CASTOR.    -   RCA will store the PUDI and the public key part of the        consistency key-pair. RCA generates a CASTOR root certificate        (Type-VI) for the CASTOR, comprising the “Cert.LicenseID”. The        public key part of the consistency key-pair is not listed in any        public available certificate.    -   The CASTOR root certificate (Type-VI) is then transferred into        the CASTOR chip and is stored therein (in a safe manner) to        prevent manipulation of same.    -   Furthermore, the CASTOR chip generates its PIO (Private-I/O) key        (also denoted I/O or umbrella key), which is a symmetric key        (e.g. a 256 bit AES symmetric key) used for encrypted data        read/write operations (“umbrella encryption”) through the        I/O-interface to CASTOR chip-external memory. The PIO key may be        generated as part of the on-chip key and identification        generation process but may also be generated (or destroyed) by        the CASTOR chip later within a setup process (e.g. triggered by        the user).

The user setup process may for example also allow the user to decidewhether his/her CASTOR should be operated more secure or with morerecovery options in case of a CASTOR chip defect. (Please note, thisuser setup process is not identical to the validation initializationprocedure described above, which is only executed once in a CASTORlifetime at the RCA site). If the user decides for a more secureoperation, the PIO key will never be exposed outside the CASTOR. In caseof a CASTOR defect it won't be possible to recover electronic tokes andall other encrypted data of the transaction system having been subjectto the umbrella encryption and being stored outside the CASTOR.

In case the user has chosen the operation with more recovery options, anencrypted copy of the PIO key will be stored in the external storage.The storage of that specific PIO can be done without umbrella protectionto allow a later recovery of same. This storage of the encrypted PIO keyis the only exception to the “on-the-fly” umbrella protection, whichisn't effective herein. The PIO key is encrypted by using the followingscheme:

-   1. The CASTOR uses the RCA's public key for encryption purposes    (K_(D:RCA:TYPE-VI,pub)) to encrypt the PIO key (even while breaking    the naming convention as this key formally is marked to be of type    decryption key). The RCA's public key used for the encryption is    comprised in the CASTOR root certificate (Type-VI certificate)    particularly for recovery purposes and is the same for all CASTORs.    This encryption makes it impossible for any third party to recover    the PIO key.-   2. The CASTOR then uses the secret key part of its consistency    key-pair (K_(S:CASTOR:CON,prv)) to encrypt the result of the    previous encryption process (i.e. the encrypted PIO key). This    encryption with the secret key part of its consistency key-pair    essentially proves the PIO key to be the specific CASTOR's one. The    public key part of the CASTORs consistency key-pair is known to the    RCA from the on-chip key and identification generation process.-   3. The CASTOR then uses another RCA's public key for signing    purposes (K_(S:RCA:TYPE-VI,pub)) to sign the previously    twice-encrypted PIO key (see steps 1 and 2). This RCA's public key    for signing purposes is also comprised in the CASTOR root    certificate (Type-VI) for recovery purposes, and is the same for all    CASTORs. This fixes the previously done encryption process and makes    it impossible to disturb the security process for any third party.

In case the CASTOR chip is defective while the external memory remainsintact (which may be highly probable if using a RAID-1 storage concept)the electronic tokens and the other transaction related data are storedencrypted by the PIO key in the memory (“the umbrella protection”)together with the PIO key encrypted using the above procedure. The RCAmay recover the electronic tokens and the transaction system relateddata for a user by recovering the PIO key from the memory and decryptingsame (in case the signature can be validated) and then decrypt theelectronic tokens and transaction system related data using therecovered PIO key—however, only if the user agreed to choose theoperation mode with a recovery operation prior the failure incident.

Digital Wallet (eWallet) Design system Considerations and Annotations

As shown within FIG. 18, the security design strategy of the exemplaryeWallet can be strengthens using the principle of logical isolation forpotential points of interests for an attacker. The System-on-Chip (SoC)functionality can be split into two sections, a secured section and anunsecured section. Hard wired circuitry not based on reprogrammableimplementations has to be rewired or tapped instead of changing thefunctionality by reprogramming by the attacker. That makes theapplication of logical attacks e.g. issued through software basedmeasures little useful.

Such a functional splitting makes it extremely difficult to compromisethe secured section even in case that the unsecured section is alreadyconquered (which is expected to be also very difficult, because of theneed to gain access by running own—not correctly signed—code on theprocessor core 2 (CPU, DMA and/or other processing elements) designatedas “ARM Cortex MPCore 2” in FIG. 18).

The processing core 1 (“ARM Cortex MPCore 1” in FIG. 18) of the securedsection is able to access all SoC areas, including the local memories ofits own and the unsecured section. In the reverse direction, this maynot be true: processing core 2 is not able to access resources at thesecured section. This may be prohibited for example by a hardware designdone in the above mentioned way, so that logical and physicalrestrictions cannot be changed during the whole lifetime of the eWallet.

In one embodiment of the invention, after a (power-on) RESET of theeWallet the processing core 1 within the secured area should be startedfirst, while in the meantime the processing core 2 within the unsecuredarea has to be captured in a RESET condition. After completing theboot-phase of the secured area processing core 1, the other processingcore 2 will be bootstrapped too (i.e. the RESET condition will bereleased by the processing core 1).

IPL- and Boot-Loader Programming

The question of eWallet platform programming is of important meaningwith respect to the production and/or to repairs and from a generalsecurity setup point. The two following aspects of the programming ofthe eWallet are considered exemplarily:

-   -   The ASIC product serialization and authentication process        through certification.    -   The ASIC's equipment with firmware (embedded operation system,        application, other code, etc.).

Both steps are based on digital certificates (also referred to asdigital signed data objects), which are injected into the internaleWallet's non-volatile memory cells of the ASIC. The usage of a digitalsignature inhibits any unwanted platform utilization of the eWallet forany third party.

As part of the production process of the eWallet ASIC at the fab, thecertificates required for proper functioning and the firmware will betransferred into the ASIC by using the [c]JTAG port. Both may besupplied by the copyright owner of the eWallet ASIC. This may occur notnecessarily in a secured environment. A first time programming of theeWallet would be useful with a quality assurance (QA) process requiredfor testability and design for manufacturing goals. Another feature isthe safeguarding of quality and transportation of the chips into theirsafe IP protection harbor. Only inside this secured area (a controlledenvironment) the activation of the ASIC internals security subsystemsshould take place. This strategy forecloses the possibility of amidnight shadow production and just in the case it happens it makes itsenseless, because these ASICS are non-functional for their intendedusage (due to the provided certificates and firmware code just fortesting).

The firmware content of the ASIC internal non-volatile memories may beprotected through the existence of a prefix-certificate (and possibleother trailing ones) in the binary images. After the event of a chipRESET, the CPU within the unsecured section may be held in that state,while the processing core within the secured section starts executing ahardwired program called IPL-Bootstrap-Routine. The IPL-BootstrapRoutine is no ROM-code, no non-volatile RAM-based code but a hardwiredgate-based and thus unalterable code being set in stone (e.g. asmicrocode). A constituent part of this (microcode) is inter alia one ormore testing signatures used within an unalterable step of recalculationbefore starting any code execution (e.g. boot code). The signatures maybe constructed in a way that one of them is enough to “open the door”for the boot process. If after a RESET the result of such a calculationdoes not match with the certificates as mentioned above, the processingunit will not jump into the boot code (no boot). Only in case of successthe bootstrap loader from the ASIC internal non-volatile memory will bestarted.

The content of all non-volatile memories (in the case there are morethan one) of the eWallet chip may be readable and writeable from theASICs outside. This is no security problem, because of the use ofcertificates. Any cloning attack will be a very difficult task, sinceall firmware images may be encrypted using the specific individual(cocoon PUF-based) fingerprint of the chip constituting, influencing andinterwoven with the digital signature used for safeguarding thecertificates.

In some embodiments, the user may be enabled to update the firmware ofthe eWallet ASIC using any external host (e.g. a PC, Smartphone areanything else able to realize host operations) using availablecommunication interfaces within the local firmware. Of course, the ASICfirmware allows only correctly signed and optionally only newer firmwareimages to be installed on the eWallet. The up-to-dateness of thefirmware could be for example tested by checking date & time and/orversion & revision tags of the firmware. In case of a signature mismatchor rollback due to the detecting a “newer” firmware being installed onthe eWallet, the eWallet ASIC simply refused the update. In case ofsuccess, the updated firmware is loaded into a RAM portion of theeWallet ASIC and may be transliterated (the firmware will be transcodedfrom the transport protection code to the eWallet security system, e.g.a cocoon PUF fingerprint-based security based one—if necessary), and maythen be transferred to an internal non-volatile memory as functional(new) updated replacement of the (old and outdated) former firmware. Theupdate of the firmware may be done in a safe way to survive any powerinterruption within the procedure.

The IPL Boot offers a minimized loading of the operation system anddiagnosing functionality, which is able to test the local chip (hard-and software) functionality within the ASIC concerning all importantfeatures in an automatic way (Build-In-System-Test, BIST).

What is claimed is:
 1. A method for manufacturing a tamper-protectedsemiconductor module comprising steps: providing a hardware modulecomprising one or more emitters for emitting a predetermined excitationthat can be measured as a physical measurand, and one or more sensorsfor sensing the back-scatter of the excitation in form of the physicalmeasurand in a contactless and/or contact-based manner, and housing thehardware module in a cocoon to obtain the tamper-protected semiconductormodule, wherein the cocoon influences the back-scatter of the excitationsensed by the sensors in a way unique to the tamper-protectedsemiconductor module.
 2. The method according to claim 1, wherein thestep housing comprises: enclosing the hardware module in a pottingmaterial, and encapsulating the hardware module enclosed by the pottingmaterial with a shielding to prevent interference with the predeterminedexcitation from the outside of the tamper-protected semiconductormodule.
 3. The method according to claim 2, further comprising the stepof producing the shielding is using one or more of the followingtechnical processes for producing different layers of shielding materialon the inside and/or outside of the cocoon surface: sintering one ormore shielding materials, sputtering one or more shielding materials,—spraying, spritzing or bubbling one or more shielding materials,coating and/or plating one or more shielding materials, immersing in oneor more baths of fluids comprising the shielding material, evaporatingone or more shielding materials, electroless plating one or moreshielding materials, and electrolysis using one or more same shieldingmaterials.
 4. The method according to claim 1, performing a thermaltreatment of the shielding.
 5. The method according to claim 1, whereinthe potting material comprises conductive material or a conductionmaterial mixture that is influencing the back-scatter of the excitationto be sensed by the sensors provided on the hardware module, and themethod further comprises the step of providing a passivation to thehardware module.
 6. A tamper-protected semiconductor module manufacturedby a method comprising steps: providing a hardware module comprising oneor more emitters for emitting a predetermined excitation that can bemeasured as a physical measurand, and one or more sensors for sensingthe back-scatter of the excitation in form of the physical measurand ina contactless and/or contact-based manner, and housing the hardwaremodule in a cocoon to obtain the tamper-protected semiconductor module,wherein the cocoon influences the back-scatter of the excitation sensedby the sensors in a way unique to the tamper-protected semiconductormodule.